Interspecific perspective on mechanical and nonmechanical models of primate circumorbital morphology.

Linear dimensions and angular orientations of the browridge, postorbital bar, and postorbital septum were obtained from a representative series of primates and compared with variables associated with several nonmechanical and biomechanical/mechanical models put forward to explain the form and function of the circumorbital region. Analyses of the results indicate that face size is the primary determinant of variation in primate circumorbital morphology. Anteroposterior browridge thickness is correlated with neural-orbital disjunction among anthropoid primates, but not among prosimians. This difference appears related to differences in the construction of the upper face and anterior cranial fossa between prosimians and anthropoids. Little support is demonstrated for the anterior dental loading model of browridge development. Mediolateral postorbital bar width and (to a lesser degree) browridge height are correlated with neurofacial torsion during mastication and variation in masticatory muscle size. These analyses further suggest that since circumorbital structures (especially the browridges) are located the farthest away from the chewing apparatus, they are least affected by masticatory stresses.     

Tooth eruption and browridge formation

One of the most reasonable hypotheses regarding the functional significance of the browridge is that the supraorbital torus forms in response to masticatory stress during development. Oyen, Walker, and Rice (1979) have recently proposed a model that tests this hypothesis: if browridges are functionally related to masticatory stresses on the cranial vault, then changes in the biomechanics of the masticatory system ought to be reflected by changes in the browridge. To test their model they attempted to relate biomechanical discontinuities resulting from tooth eruption to episodes of bone deposition on the supraorbital tori of a developmental series of dry Papio crania. This paper reports on a parallel test of the model on a cross-sectional sample of Australian Aboriginal juvenile crania. This sample showed no relation between tooth eruption and the supraorbital surface morphology thought to be indicative of active bone deposition. It is also demonstrated that no significant relationship between tooth eruption and episodes of bone deposition is shown by the Papio sample. It is concluded that the use of small cross-sectional samples of dry crania does not provide a valid test of the model.     

Endo's stress analysis of the primate skull and the functional significance of the supraorbital region

A review of Endo's experimental and theoretical procedures and data indicates that the magnitude of the principal strains in the glabella region of both humans and gorillas are low as compared to other parts of the face. Therefore, his data do not provide support for the hypothesis that the glabella region is a highly stressed region during biting. In addition, increased levels of strain in the supraorbital region are directly related to increased levels of masticatory muscle and reaction forces, and not necessarily to anterior tooth loading as opposed to posterior tooth loading. His data also indicate that the supraorbital region in extant humans cannot be accurately modeled as a beam. These conclusions either differ from those of Endo or are not clearly presented or emphasized throughout any of Endo's papers. Therefore, we suggest that a number of investigators have made unsupported or erroneous conclusions based on Endo's work. This is particularly true for those studies that have emphasized the existence of powerful bending stress in the glabella region during incisor biting in both humans and non-human primates.     

Ontogenetic study of the supraorbital region in modern humans: a longitudinal test of the spatial model

The structural significance of the hominid supraorbital torus and its morphological variation have always been a controversial topic in physical anthropology. Understanding the function of browridge variation in living and fossil human populations is relevant to questions of human evolution. This study utilizes radiograph images to evaluate the spatial model in modern humans during ontogeny. This structural model attributes variation in the supraorbital region to the positional relationship between the neurocranium and the orbits. The relationship between measurements of the antero-posterior supraorbital length and the factors specified in the spatial model were assessed by correlation and partial correlation analyses. Growth rates were also examined to study ontogenetic trajectories and infer aspects of developmental relationships between critical variables. Results agree with previous research supporting the existence of spatial influences between the neural and orbital-upper facial regions on browridge length during ontogeny.     

The functional significance of primate mandibular form.

A stress analysis of the primate mandible suggests that vertically deep jaws in the molar region are usually an adaptation to counter increased sagittal bending stress about the balancing-side mandibular corpus during unilateral mastication. This increased bending stress about the balancing side is caused by an increase in the amount of balancing-side muscle force. Furthermore, this increased muscle force will also cause an increase in dorso-ventral shear stress along the mandibular symphysis. Since increased symphyseal stress can be countered by symphyseal fusion and as increased bending stress can be countered by a deeper jaw, deep jaws and symphyseal fusion are often part of the same functional pattern. In some primates (e.g., Cercocebus albigena), deep jaws are an adaptation to counter bending in the sagittal plane during powerful incisor biting, rather than during unilateral mastication. The stress analysis of the primate mandible also suggests that jaws which are transversely thick in the molar region are an adaptation to counter increased torsion about the long axis of the working-side mandibular corpus during unilateral mastication. Increased torsion of the mandibular corpus can be caused by an increase in masticatory muscle force, an increase in the transverse component of the postcanine bite force and/or an increase in premolar use during mastication. Patterns of masticatory muscle force were estimated for galagos and macaques, demonstrating that the ratio of working-side muscle force to balancing-side muscle force is approximately 1.5:1 in macaques and 3.5:1 in galagos during unilateral isometric molar biting. These data support the hypothesis that mandibular symphyseal fusion is an adaptative response to maximize unilateral molar bite force by utilizing a greater percentage of balancing-side muscle force.     

In vivo and in vitro bone strain in the owl monkey circumorbital region and the function of the postorbital septum

Anthropoids and tarsiers are the only vertebrates possessing a postorbital septum. This septum, formed by the frontal, alisphenoid, and zygomatic bones, separates the orbital contents from the temporal muscles. Three hypotheses suggest that the postorbital septum evolved to resist stresses acting on the skull during mastication or incision. The facial-torsion hypothesis posits that the septum resists twisting of the face about a rostrocaudal axis during unilateral mastication; the transverse-bending hypothesis argues that the septum resists caudally directed forces acting at the lateral orbital margin during mastication or incision; and the tension hypothesis suggests that the septum resists ventrally directed components of masseter muscle force during mastication and incision. This study evaluates these hypotheses using in vitro and in vivo bone strain data recorded from the circumorbital region of owl monkeys. Incisor loading of an owl monkey skull in vitro bends the face upward in the sagittal plane, compressing the interorbital region rostrocaudally and “buckling” the lateral orbital walls. Unilateral loading of the toothrow in vitro also bends the face in the sagittal plane, compressing the interorbital region rostrocaudally and buckling the working side lateral orbital wall. When the lateral orbital wall is partially cut, so as to reduce the width of its attachment to the braincase, the following changes in circumorbital bone strain patterns occur. During loading of the incisors, lower bone strain magnitudes are recorded in the interorbital region and lateral orbital walls. In contrast, during unilateral loading of the P³, higher bone strain magnitudes are observed in the interorbital region, and generally lower bone strain magnitudes are observed in the lateral orbital walls. During unilateral loading of the M², higher bone strain magnitudes are observed in both the interorbital region and in the lateral orbital wall ipsilateral to the loaded molar. Comparisons of the in vitro results with data gathered in vivo suggest that, during incision and unilateral mastication, the face is subjected to upward bending in the sagittal plane resulting in rostrocaudal compression of the interorbital region. Modeling the lateral orbital walls as curved plates suggests that during mastication the working side wall is buckled due to the dorsally directed component of the maxillary force which causes upward bending of the face in the sagittal plane. The balancing side lateral orbital wall may also be buckled due to upward bending of the face in the sagittal plane as well as being twisted by the caudoventrally directed components of the superficial masseter muscle force. The in vivo data do not exclude the possibility that the postorbital septum functions to improve the structural integrity of the postorbital bar during mastication. However, there is no reason to believe that a more robust postorbital bar could not also perform this function. Hypotheses stating that the postorbital septum originally evolved to reinforce the skull against routine masticatory loads must explain why, rather than evolving a postorbital septum, the stem anthropoids did not simply enlarge their postorbital bars. © 1996 Wiley-Liss, Inc.     

Mandibular function in Galago crassicaudatus and Macaca fascicularis: an in vivo approach to stress analysis of the mandible.

Single-element and/or rosette strain gages were bonded to mandibular cortical bone in Galago crassicaudatus and Macaca fascicularis. Five galago and eleven macaque bone strain experiments were performed and analyzed. In vivo bone strain was recorded from the lateral surface of the mandibular corpus below the postcanine tooth row during transducer biting and during mastication and ingestion of food objects. In macaques and galagos, the mandibular corpus on the balancing side is primarily bent in the sagittal plane during mastication and is both twisted about its long axis and bent in the sagittal plane during transducer biting. On the working side, it is primarily twisted about its long axis and directly sheared perpendicular to its long axis, and portions of it are bent in the sagittal plane during mastication and molar transducer biting. In macaques, the mandibular corpus on each side is primarily bent in the sagittal plane and twisted during incisal transducer biting and ingestion of food objects, and it is transversely bent and slightly twisted during jaw opening. Since galagos usually refused to bite the transducer or food objects with their incisors, an adequate characterization of mandibular stress patterns during these behaviors was not possible. In galagos the mandibular corpus experiences very little transverse bending stress during jaw opening, perhaps in part due to its unfused mandibular symphysis. Marked differences in the patterns of mandibular bone strain were present between galagos and macaques during the masticatory power stroke and during transducer biting. Galagos consistently had much more strain on the working side of the mandibular corpus than on the balancing side. These experiments support the hypothesis that galagos, in contrast to macaques, employ a larger amount of working-side muscle force relative to the balancing-side muscle force during unilateral biting and mastication, and that the fused mandibular symphysis is an adaption to use a maximal amount of balancing-side muscle force during unilateral biting and mastication. These experiments also demonstrate the effects that rosette position, bite force magnitudes, and types of food eaten have on recorded mandibular strain patterns.     

In vivo function of the craniofacial haft: the interorbital "pillar"

The craniofacial haft resists forces generated in the face during feeding, but the importance of these forces for the form of the craniofacial haft remains to be determined. In vivo bone strain data were recorded from the medial orbital wall in an owl monkey (Aotus), rhesus macaques (Macaca mulatta), and a galago (Otolemur) during feeding. These data were used to determine whether: the interorbital region can be modeled as a simple beam under bending or shear; the face is twisting on the brain case during unilateral biting or mastication; the interorbital "pillar" is being axially compressed during incisor loading and both axially compressed and laterally bent during mastication; and the interorbital "pillar" transmits axial compressive forces from the toothrow to the braincase. The strain data reveal that the interorbital region cannot be modeled as a anteroposteriorly oriented beam bent superiorly in the sagittal plane during incision or mastication. The strain orientations recorded in the majority of experiments are concordant with those predicted for a short beam under shear, although the anthropoids displayed evidence of multiple loading regimes in the medial orbital wall. Strain orientation data corroborate the hypothesis that the strepsirrhine face is twisted during mastication. The hypothesis that the interorbital region is a member in a rigid frame subjected to axial compression during mastication receives some support. The hypothesis that the interorbital region is a member in a rigid frame subjected to lateral bending during mastication is supported by the epsilon1/absolute value epsilon2 ratio data but not by the strain orientation data. The timing of peak shear strains in the medial orbital wall of anthropoids does not bear a consistent relationship to the timing of peak shear strain in the mandibular corpus, suggesting that bite force is not the only external force influencing the medial orbital wall. Strain orientation data suggest the existence of two distinct loading regimes, possibly associated with masseter or medial pterygoid contraction. Regardless of the loading regime, all taxa showed low strain magnitudes in the medial orbital wall relative to the anterior root of the zygoma and the mandibular corpus. The strain gradients documented here and elsewhere suggest that, in anthropoids at least, local effects of external forces are more important than a single global loading regime. The low strain magnitudes in the medial orbital wall and in other thin bony plates around the orbit suggest that these structures are not optimally designed for resisting feeding forces. It is hypothesized that their function is to provide rigid support and protection for soft-tissue structures such as the nasal epithelium, the brain, meninges, and the eye and its adnexa. In contrast with the face of Otolemur, which appears to be subjected to a single predominant loading regime, anthropoids may experience different loading regimes in different parts of the face. This implies that the anthropoid and strepsirrhine facial skulls might be optimized for different functions.     

Biomechanical scaling of the hominoid mandibular symphysis

Experimental investigation of mandibular bone strain in cercopithecine primates has established that the mandible is bent in the transverse plane during the power stroke of mastication. Additional comparative work also supports the assumption that the morphology of the mandibular symphysis is functionally linked to the biomechanics of lateral transverse bending, or "wishboning" of the mandibular corpus. There are currently no experimental data to verify that lateral transverse bending constitutes an important loading regime among hominoid primates. There are, however, allometric models from cercopithecoid primates that allow prediction of scaling patterns in hominoid mandibular dimensions that would be consistent with a mechanical environment that includes wishboning as a significant component. This study uses computed tomography (CT) scans to visualize cortical bone distribution in the anterior corpus of a sample of four genera of extant hominoids. From the cortical bone contours, area properties of the mandibular symphysis are calculated, and these variables are subjected to an allometric analysis to detect whether scaling of jaw dimensions are consistent with a wishboning loading regime. Scaling of the hominoid symphysis recalls patterns observed in cercopithecoid monkeys, which lends indirect support for the hypothesis that wishboning is an integral part of the masticatory loading environment in living apes. Inclination of the symphysis, rather than changes in cross-sectional shape or development of the superior transverse torus, represents a morphological solution for minimizing the potentially harmful effects of wishboning in the jaws of these primates.     

Masticatory loading and bone adaptation in the supraorbital torus of developing macaques

Research on the evolution and adaptive significance of primate craniofacial morphologies has focused on adult, fully developed individuals. Here, we investigate the possible relationship between the local stress environment arising from masticatory loadings and the emergence of the supraorbital torus in the developing face of the crab‐eating macaque Macaca fascicularis. By using finite element analysis (FEA), we are able to evaluate the hypothesis that strain energy density (SED) magnitudes are high in subadult individuals with resulting bone growth in the supraorbital torus. We developed three micro‐CT‐based FEA models of M. fascicularis skulls ranging in dental age from deciduous to permanent dentitions and validated them against published experimental data. Applied masticatory muscle forces were estimated from physiological cross‐sectional areas of macaque cadaveric specimens. The models were sequentially constrained at each working side tooth to simulate the variation of the bite point applied during masticatory function. Custom FEA software was used to solve the voxel‐based models and SED and principal strains were computed. A physiological superposition SED map throughout the face was created by allocating to each element the maximum SED value from each of the load cases. SED values were found to be low in the supraorbital torus region throughout ontogeny, while they were consistently high in the zygomatic arch and infraorbital region. Thus, if the supraorbital torus arises to resist masticatory loads, it is either already adapted in each of our subadult models so that we do not observe high SED or a lower site‐specific bone deposition threshold must apply. Am J Phys Anthropol, 2009. © 2008 Wiley‐Liss, Inc.     

Masticatory stress, orbital orientation and the evolution of the primate postorbital bar

A postorbital bar is one of a suite of derived features which distinguishes basal primates from their putative sister taxon, plesiadapiforms. Two hypotheses have been put forward to explain postorbital bar development and variation in circumorbital form: the facial torsion model and visual predation hypothesis. To test the facial torsion model, we employ strain data on circumorbital and mandibular loading patterns in representative primates with a postorbital bar and masticatory apparatus similar to basal primates. To examine the visual predation hypothesis, we employ metric data on orbit orientation in Paleocene and Eocene primates, as well as several clades of visual predators and foragers that vary interspecifically in postorbital bar formation.A comparison of galago circumorbital and mandibular peak strains during powerful mastication demonstrates that circumorbital strains are quite low. This indicates that, as in anthropoids, the strepsirhine circumorbital region is excessively overbuilt for countering routine masticatory loads. The fact that circumorbital peak-strain levels are uniformly low in both primate suborders undermines any model which posits that masticatory stresses are determinants of circumorbital form, function and evolution. This is interpreted to mean that sufficient cortical bone must exist to prevent structural failure due to non-masticatory traumatic forces. Preliminary data also indicate that the difference between circumorbital and mandibular strains is greater in larger taxa.Comparative analyses of several extant analogs suggest that the postorbital bar apparently provides rigidity to the lateral orbital margins to ensure a high level of visual acuity during chewing and biting. The origin of the primate postorbital bar is linked to changes in orbital convergence and frontation at smaller sizes due to nocturnal visual predation and increased encephalization. By incorporating in vivo and fossil data, we reformulate the visual predation hypothesis of primate origins and thus offer new insights into major adaptive transformations in the primate skull.     

Strain in the galago facial skull

Little experimental work has been directed at understanding the distribution of stresses along the facial skull during routine masticatory behaviors. Such information is important for understanding the functional significance of the mammalian circumorbital region. In this study, bone strain was recorded along the dorsal interorbit, postorbital bar, and mandibular corpus in Otolemur garnettii and O. crassicaudatus (greater galagos) during molar chewing and biting. We determined principal-strain magnitudes and directions, compared peak shear-strain magnitudes between various regions of the face, and compared galago strain patterns with similar experimental data for anthropoids. This suite of analyses were used to test the facial torsion model (Greaves [1985] J Zool (Lond) 207:125-136; [1991] Zool J Linn Soc 101:121-129; [1995] Functional morphology in vertebrate paleontology. Cambridge: Cambridge University Press, p 99-115). A comparison of galago circumorbital and mandibular peak strains during powerful mastication indicates that circumorbital strains are very low in magnitude. This demonstrates that, as in anthropoids, the strepsirhine circumorbital region is highly overbuilt for countering routine masticatory loads. The fact that circumorbital peak-strain magnitudes are uniformly low in both primate suborders undermines any model that emphasizes the importance of masticatory stresses as a determinant of circumorbital form, function, and evolution. Preliminary data also suggest that the difference between mandibular and circumorbital strains is greater in larger-bodied primates. This pattern is interpreted to mean that sufficient cortical bone must exist in the circumorbital region to prevent structural failure due to nonmasticatory traumatic forces. During unilateral mastication, the direction of epsilon(1) at the galago dorsal interorbit indicates the presence of facial torsion combined with bending in the frontal plane. Postorbital bar principal-strain directions during mastication are oriented, on average, very close to 45 degrees relative to the skull's long axis, much as predicted by the facial torsion model. When chewing shifts from one side of the face to the other, there is a characteristic reversal or flip-flop in principal-strain directions for both the interorbit and postorbital bar. Although anthropoids also exhibit an interorbital reversal pattern, peak-strain directions for this clade are opposite those for galagos. The presence of such variation may be due to suborder differences in relative balancing-side jaw-muscle force recruitment. Most importantly, although the strain-direction data for the galago circumorbital region offer support for the occurrence of facial torsion, the low magnitude of these strains suggests that this loading pattern may not be an important determinant of circumorbital morphology.     

Mandibular biomechanics and temporomandibular joint function in primates.

There is disagreement as to whether the mandibular condyles are stress-bearing or stress-free during mastication. In support of alternative models, analogies have been drawn with Class III levers, links, and couple systems. Physiological data are reviewed which indicate that maximum masticatory forces are generated when maxillary and mandibular teeth are in contact, and that this phase lasts for over 100 msec during many chewing strokes. During this period, the mandible can be modeled as a beam with multiple supports. Equations of simple beam theory suggest that large condylar reaction forces are present during mastication. With unilateral molar biting in man, the total condylar reaction force may be over 75% of the bite force. Analysis of a frontal projection demonstrates that up to 80% of the total condylar reaction force is borne by the contralateral (balancing side) condyle during unilateral molar biting. A comparison of human, chimpanzee (P. troglodytes), spider monkey (A. belzebuth), and macaque (Macaca sp.) morphology indicates that the frugivorous chimpanzee and spider monkey have a relatively lower condylar reaction force than the omnivorous macaque or man during molar biting. The percentage reaction force during incisal biting is lower in man than in the other primates, and lower in the frugivorous primates than in the macaque.     

Occlusal forces and mandibular bone strain: Is the primate jaw “overdesigned”?

Finite element modelling of the function of the periodontium and surrounding alveolar bone suggests these tissues are subjected to unusually large strains in comparison with the bone of the basal mandibular corpus. These studies, in addition to certain experimental investigations, have led to the suggestion that the strains experienced in the basal mandibular corpus are not functionally important. Under this view, size and shape of the basal corpus are not functionally linked to masticatory forces. Since previous comparative investigations have been premised on the assumption that masticatory strains in the basal corpus are functionally important, the assertion that masticatory stresses are concentrated primarily in the alveolar process undermines the credibility of this body of work. The hypothesis that the biomechanical effects of masticatory forces are localized in the alveolar process can be evaluated by reference to a number of bone strain investigations, as well as through consideration of current understanding of bone biology and behavior. Experimental studies indicate that the effects of occlusal forces during mastication are quite apparent in alveolar bone, although relatively large strains are also observed in regions well-removed from a loaded alveolus. It is also apparent that both alveolar and basal mandibular bone are subject to bending and twisting strains associated not only with occlusal forces, but also with muscular and condylar reaction forces. The result is that strain levels in alveolarvs.basal bone may be roughly similar, in contradiction to some published theoretical models. Based on empirical evidence and theoretical considerations, it is premature to conclude that mandibular corpus size and shape are not functionally linked to the biomechanics of chewing and biting.     

Experimental analysis of temporomandibular joint reaction force in macaques

Mandibular bone strain in the region immediately below the temporomandibular ligament was analyzed in adult and sub-adult Macaca fascicularis and Macaca mulatta. Following recovery from the general anesthetic, the monkeys were presented food objects, a wooden rod, or a specially designed bite-force transducer. Bone strain was recorded during incisal biting and mastication of food, and also during isometric biting of the rod and/or the transducer. The bone strain data suggest the following: The macaque TMJ is loaded by a compressive reaction force during the power stroke of mastication and incision of food, and during isometric molar and incisor biting. TMJ reaction forces are larger on the contralateral side during both mastication and isometric molar biting. Patterns of ipsilateral TMJ reaction force in macaques during isometric biting vary markedly in response to the position of the bite point. During biting along the premolars or first two molars a compressive reaction force acts about the ipsilateral TMJ; however, when the bite point is positioned along the M3, the ipsilateral TMJ has either very little compressive stress, no stress, or it is loaded in tension.     

Telemetry System for Assessing Jaw-Muscle Function in Free-ranging Primates

In vivo laboratory-based studies describing jaw-muscle activity and mandibular bone strain during mastication provide the empirical basis for most evolutionary hypotheses linking primate masticatory apparatus form to diet. However, the laboratory data pose a potential problem for testing predictions of these hypotheses because estimates of masticatory function and performance recorded in the laboratory may lack the appropriate ecological context for understanding adaptation and evolution. For example, in laboratory studies researchers elicit rhythmic chewing using foods that may differ significantly from the diets of wild primates. Because the textural and mechanical properties of foods influence jaw-muscle activity and the resulting strains, chewing behaviors studied in the laboratory may not adequately reflect chewing behaviors of primates feeding in their natural habitats. To circumvent this limitation of laboratory-based studies of primate mastication, we developed a system for recording jaw-muscle electromyograms (EMGs) from free-ranging primates so that researchers can conduct studies of primate jaw-muscle function in vivo in the field. We used the system to record jaw-muscle EMGs from mantled howlers (Alouatta palliata) at Hacienda La Pacifica, Costa Rica. These are the first EMGs recorded from a noncaptive primate feeding in its natural habitat. Further refinements of the system will allow long-term EMG data collection so that researchers can correlate jaw-muscle function with food mechanical properties and behavioral observations. In addition to furthering understanding of primate feeding biology, our work will foster improved adaptive hypotheses explaining the evolution of primate jaw form.     

Mandibular growth and function in Archaeolemur

Ontogenetic changes in the morphology of the mandibular symphysis are described in Archaeolemur so as to infer the functional significance of symphyseal fusion in this subfossil Malagasy lemur. The first regions of the symphysis to show a more complex morphology were the lower and anterior borders of the joint and, to a lesser extent, the lingual borders of the superior and inferior transverse tori. During growth, these regions became increasingly rugose and encroached upon a centrally located, smooth, “oval” region, which may have been a principal pathway for neurovascular structures communicating with the unfused joint. In subadults, the symphysis was completely fused except for the lingual surface of the inferior transverse torus, where a patent suture and potential space were present between dentaries. Thus, in Archaeolemur there was an age- and size-related pattern of increased symphyseal ossification or fusion that was complete by adulthood. The morphology of the interlocking bony processes and the sequence of ossification in the symphysis suggest that increased dorsoventral shear stress during mastication was the most likely determinant of symphyseal fusion in Archaeolemur: The allometric pattern of greater symphyseal fusion may be linked to the presence of relatively greater dorsoventral shear in adults due to an increased recruitment of balancing-side jaw-muscle force. There is little indication that the symphysis of juvenile Archaeolemur was buttressed to resist forces associated with “wishboning” during mastication or vertical bending during incision. Our observations, as well as those of others, suggest that symphyseal fusion in primates occurs initially as a response to increased dorsoventral shear during mastication. Therefore, wishboning stress might only become a major determinant of symphyseal form and function in those taxa that develop a fused symphysis to counter increased dorsoventral shear. © 1994 Wiley-Liss, Inc.     

Pharyngeal mastication and food transport in the carp (Cyprinus carpio L.): A cineradiographic and electromyographic study

Cyprinids constitute the largest fish family and are characterized by their pharyngeal teeth. The masticatory mechanism is still poorly understood. The complex of structures that determine the movements of pharyngeal teeth and chewing pad in the carp (Cyprinus carpio L.) is analyzed. Activities in 16 head muscles of a free-swimming carp were recorded. X-ray cinerecordings, synchronized with electromyograms, were made of the intake, transport, mastication, and deglutition of radiopaque food pellets. Metal markers allowed a detailed movement analysis. Masticatory cycles are bilaterally synchronous and show distinct crushing and grinding patterns. Direct masticatory muscles that suspend and connect the pharyngeal bones steer and stabilize the masticatory movements. Baudelot's ligament, between skull and pectoral girdle, is applied as fulcrum, effects a crucial shift of the rotation axis of the pharyngeal jaw, and transforms crushing into grinding; simultaneous abduction lengthens the grinding stroke. Body muscles supply indirectly the power for mastication; they also appear to be regulated more distantly. The epaxial muscles lift the skull and thereby the levators of the pharyngeal bones, thus transmitting high forces to the teeth. They also stretch the levator of the bone as soon as occlusion is reached and thus optimize its production of forces during grinding. The hypaxial muscles retract the pharyngeal bones indirectly during grinding and power the teeth in sliding. The chewing pad, previously assumed to be motionless, rotates rostroventrad with the skull and intensifies grinding. Respiration and mastication are mutually related. The extensive movements of the pharyngeal bones are permitted only by the simultaneous expansion of the buccopharynx and a slide-coupling in the branchial floor. Muscular pads that line the pharynx are shown to transport food toward the teeth. The constrictor pharyngis effects deglutition. Natural food, intestinal contents, and feces of the carp were analyzed with respect to the capacity for distinct masticatory operations. During the experiments pellets, barley, and worms were fed. The carp is specialized for polyphagy and this appears to be based on the profiles of the heterodont teeth rather than on drastic changes in the two preprogrammed activity patterns. Comparison of the pharyngeal jaw system in the carp and higher teleosts emphasizes the structural design for the application of large forces in this cyprinid.     

The Nature of the Reaction of the Colon Organism on Endo's Medium

Bacterium coli was grown in a medium of a composition similar to Endo's medium with the exception that the sulfite, fuchsin and agar were left out. When the fuchsin-sulfite mixture was added after 24, 36 and 48 hours, or after 3 and 10 days of incubation at 37°C., no reaction appeared. The substance that causes the reaction is only formed when the bacteria are grown in the presence of sulfite. When picric acid and ether are added to the red compound produced by the colon organism in Endo's medium and the mixture shaken, the color remains in the watery layer and thus the dye color that appears in the medium is not restored fuchsin, but a new substance. The work confirms Neuberg's and Nord's theory that the Endo medium acts as a trapping agent for the intermediate product, acetaldehyde, which causes the Endo reaction.     

Temporalis and masseter muscle function during incision in macaques and humans

Most previously published electromyographic (EMG) studies have indicated that the temporalis muscles in humans become almost electrically quiet during incisai biting. These data have led various workers to conclude that these muscles may contribute little to the incisai bite force. The feeding behavior and comparative anatomy of the incisors and temporalis muscles of certain catarrhine primates, however, suggest that the temporalis muscle is an important and powerful contributor to the bite force during incision. One purpose of this study is to analyze the EMG activity of the masseter and temporalis muscles in both humans and macaques with the intention of focusing on the conflict between published EMG data on humans and inferences of muscle function based on the comparative anatomy and behavior of catarrhine primates. The EMG data collected from humans in the present study indicate that, in five of seven subjects, the masseter,anterior temporalis, and posterior temporalis muscles are very active during apple incision (i.e., relative to EMG activity levels during apple and almond mastication). In the other two human subjects the EMG levels of these muscles are lower during incision than during mastication, but in no instance are these muscles ever close to becoming electrically quiet. The EMG data on macaques indicate that, in all six subjects, the masseter, anterior temporalis, and posterior temporalis muscles are very active during incision. These data are in general agreement with inferences on muscle function that have been drawn from the comparative anatomy and behavior of primates, but they do not agree with previous experimental data. The reason for this disagreement is probably due to differences in the experimental procedure. In previous studies subjects simply bit isometrically on their incisors and the resulting EMG pattern was compared to the pattern associated with powerful clenching in centric occlusion. In the present study the subjects incised into actual food objects, and the resulting EMG pattern was compared to the pattern associated with mastication of various foods. It is not surprising that these two procedures result in markedly different EMG patterns, which in turn result in markedly different interpretations of jaw-muscle function. In an attempt to explain the evolution of the postorbital septum in anthropoids, it has been suggested that the anterior temporalis is more active than the masseter during incision (Cachel, 1979). The human and macaque EMG data do not support this hypothesis; during incision, the two muscles show no consistent differences in humans and the masseter appears to be in fact more active than the anterior temporalis in macaques.