[Biomechanics of the jaw apparatus in the horn-shark (Heterodontus portusjacksoni = Heterodontus philippi)]. / Die Biomechanik des Kieferapparates beim Stierkopfhai (Heterodontus portusjacksoni = Heterodontus philippi)

The constructional morphology of the jaw apparatus in the horn-shark Heterodontus has been investigated. The origines and insertions of the jaw muscles have been delimited and the lines of action were determined. The individual muscles have been weighed, in order to get on the basis of their masses an estimate of the forces which are exerted by the components of the jaw musculature. The mandibular joints, the occlusion of the jaws, morphological details of the upper and lower jaws as well as the "subodontium" have been subject to macroscopic and microscopic study. The joint between palatoquadrate and mandibular consists of a medial and a lateral compartment. Only hinge movements are possible. If the jaws are closed, the contacts between upper and lower jaws are confined to the large crushing teeth of only one or two tooth families. The teeth are fixed to the jaw cartilage by ligamentous structures. Three layers can be discerned histologically: The uppermost layer, beneath the bases of the teeth, is composed of the fibrae interdentales and of the fibrae subbasales. By these, the individual teeth are firmly connected to form a continuous pavement. - The middle layer is characterised by the great number of cell nuclei. - The fibre system which constitutes the lowermost layer is arranged according to its function. The subdental layer of fibrous tissue grows faster than the one adjacent to the jaws. Thus a particular growth structure is formed. - Two constructive principles are realised: 1. The biting forces or "loads", applied to one or two teeth, are split and distributed on all teeth of the same family which are lingual of the loaded one. 2. As in the thecodont mammals, the compressive biting (= occlusal) force is transformed into a tensile force by the tooth-fixing apparatus and by the shapes of the jaws and this is sustained by fibrous structures. Magnitude and directions of the stresses which appear in the upper and lower jaw during biting are compared with stress patterns evoked in consoles and in beams of technical constructions. The moments of resistence have been calculated for 9 or 8 cross sections, respectively, through the upper and the lower jaw. The forms of both jaws are comparable to a beam of equal strength on two supports, namely, bitten object and joints; while the load is represented by the muscular force. Where the bending moments are at a maximum, opposite to the muscle insertions, a strong ligamentous reinforcement of the perichondrium extends parallel to the tooth rows and fades out towards the joint and towards the symphysis. The strength properties of cartilage and of collagenous fibres in the species under consideration have been measured. The lines of action of the jaw muscles intersect in the area where the moments of resistence reach their maximum. The largest crushing teeth are located in this same section, so that the muscles exert most force on the teeth, while the joint is exposed to only moderate compression.

Aspects of masticatory form and function in common tree shrews, Tupaia glis.

Tree shrews have relatively primitive tribosphenic molars that are apparently similar to those of basal eutherians; thus, these animals have been used as a model to describe mastication in early mammals. In this study the gross morphology of the bony skull, joints, dentition, and muscles of mastication are related to potential jaw movements and cuspal relationships. Potential for complex mandibular movements is indicated by a mobile mandibular symphysis, shallow mandibular fossa that is large compared to its resident condyle, and relatively loose temporomandibular joint ligaments. Abrasive tooth wear is noticeable, and is most marked at the first molars and buccal aspects of the upper cheek teeth distal to P2. Muscle morphology is basically similar to that previously described for Tupaia minor and Ptilocercus lowii. However, in T. glis, an intraorbital part of deep temporalis has the potential for inducing lingual translation of its dentary, and the large medial pterygoid has extended its origin anteriorly to the floor of the orbit, which would enhance protrusion. The importance of the tongue and hyoid muscles during mastication is suggested by broadly expanded anterior bellies of digastrics, which may assist mylohyoids in tensing the floor of the mouth during forceful tongue actions, and by preliminary electromyography, which suggests that masticatory muscles alone cannot fully account for jaw movements in this species.

Muscles of the masticatory apparatus in two genera of hyraces (Procavia and Heterohyrax).

Subungulate hyraces are similar to the condition assumed to have characterized primitive ungulates and subungulates by virtue of their small body size, relatively unspecialized cranial and postcranial anatomy, and primitive type of lophodont dentition. The muscles of mastication of Procavia habesssinica and Heterohyrax brucei are here compared with those of other mammals, both with ungulates, as an example of more specialized mammals, and with opossums, as an example of more generalized mammals, to determine aspects of hyrax myology that represent the retention of a condition primitive for herbivorous mammals. The masticatory muscles of hyraces retain the primitive ungulate/subungulate condition in the large, complexly subdivided temporalis, and in the enlarged, pinnated, bilayered medial pterygoid. The medial pterygoid originates from the pterygoid hamulus, a condition that may also be primitive for this assemblage. The large complex superficial masseter is derived compared with the condition in ruminant artiodactyls, but may represent the condition primitive for perissodactyls. The architectural modifications of this muscle in hyraces may represent adaptations to allow a wide gape threat display. Hyraces possess a posterior belly of the digastric alone, paralleling the condition in some perissodactyls. They possess a large and complexly subdivided styloglossus, which may be a shared derived character of subungulates. Hyraces are unique among ungulates and subungulates in the extreme reduction of the anterior hyoid cornua, and may be unique among mammals in the development of paired lingual processes from the ceratohyal ossifications.

Morphological investigation into functions of the jaw symphysis in carnivorans.

The hemimandibles in carnivorans may be united in various ways at the symphysis menti. The symphysis may contain a readily flexible joint that permits a moderate amount of independent movement of the hemimandibles. This type of symphyseal union is primitive for and widely distributed in extant carnivorans. In other carnivorans, the symphysis is patent but allows slight or essentially no independent movement of the hemimandibles. Finally, the hemimandibles may be rigidly united by synostosis of the symphysis. The morphology, movement and, insofar as possible, function of these types of symphyses are described.

Mandibular movement and muscle activity during mastication in the guinea pig (Cavia porcellus).

Optoelectronic analysis of mandibular movement and electromyography (EMG) of masticatory muscles in Cavia porcellus indicate bilateral, unilateral, and gnawing cycles. During bilateral and unilateral cycles, the mandibular tip moves forward, lateral, and down during the lingual phase of the power stroke to bring the teeth into occlusion. EMG activity is generally asymmetric, with the exception of activity of the temporalis muscle during bilateral cycles. During gnawing cycles, the mandible moves in an anteroposterior direction that is opposite that during bilateral and unilateral chew cycles. Bilateral and unilateral cycles of pellets were significantly longer than carrot. With the exception of the width of bilateral cycles, the magnitude of cycle width, length, and height during the mastication of carrots was greater than that during the mastication of pellets. Significant differences exist between EMG durations during mastication of pellets and carrots. The lateral pterygoid displays continuous activity during gnawing cycles. Significant differences also exist in the duration of EMG activity between the working and balancing side during all three cycle types. High level of activity of balancing side temporalis and anterior belly of digastric (ABD) during bilateral cycles occurs during rotation and depression of the mandible during the power stroke. The temporalis apparently provides a "braking" or compensatory role during closing and power strokes. Differences between Cavia masticatory patterns and those shown by Rattus and Mesocricetus are apparently due to differences in dental morphology, occlusal relationships, and, possibly, the poorly developed temporalis in Cavia. The large number and wide diversity of rodent groups afford students of mammalian mastication an opportunity to investigate and compare different masticatory specializations.

Oral food processing in two herbivorous lizards, Iguana iguana (Iguanidae) and Uromastix aegyptius (Agamidae).

The anatomy and function of the feeding apparatus in Iguana iguana and Uromastix aegyptius were studied by dissection, cinematic and cineradiographic techniques. The feeding behavior of these species differs from that of insectivorous lizards in the cropping action involves movement of both the upper jaw around the atlantooccipital joint and the lower jaw around the mandibular joint; and in Uromastix only, streptostylic movement of the quadrate. Often movements of the whole head play a supplementary role in the cropping action. In both species the feeding apparatus has been modified to facilitate cropping. In Iguana the pleurodont dentition is multicusped and laterally compressed. Each tooth forms a shearing blade whose function does not require contact with other teeth. In Uromastix the dentition is acrodont and the cheek teeth are massive and lack cusps. Occlusion is necessary for shearing plant material. The skull system of Uromastix also has a number of modified structures which allow protraction and retraction of the lower jaw to facilitate cropping while maintaining a gape equivalent to that in Iguana. It is suggested that the differences in the feeding apparatus between Iguana and Uromastix are attributable to differeces in the mode of tooth replacement and implantation.

Dental aspects of maxillofacial injuries.

The chief progress of maxillofacial surgery in recent years has been the preservation and the retention of functional tissue. The recent advances in microvascular and reimplantation surgery are examples. Similarly the preservation of the masticatory apparatus is best served by the retention of all viable components. Additional benefits derived from this approach include the facilitation with alignment of the bony fragments, proper occlusion of the teeth and maintenance of the vertical relationship between the upper and lower jaws; in addition to the restoration of appearance and the functions of mastication and speech. Although injuries of the teeth and their supporting stuctures at first glance may seem to be simple problems, frequently they turn out to be complicated. The most common injury is partial avulsion of the alveolar bone with loss of attached teeth, which may produce a severe defect in the jaw bone. During healing there is additional resorbtion of alveolar bone, which is quite a normal physiologic phenomenon. Therefore, the eventual defect is likely to be more severe yet. For this reason more concern should be shown for these lesser injuries.

A factor analysis of traits as measured by COCSTOC-MOT to identify occlusal trait combinations.

Occlusal conditions which vary significantly from accepted social norms for dental appearance may socially handicap an individual. An instrument is required by which socially unacceptable occlusal conditions can be identified by members of the public who have internalized society's norms for acceptable dental appearance. The factors identified in this paper make it possible to select a reasonable number (100) of models for constructing a measurement instrument which depicts the full range of occlusal patterns found in a natural population for use in assessing the social acceptability of occlusal conditions. This paper presents the methodology and process used to identify individual occlusal traits which tend to occur together in trait combinations or independently and their range from "normal" to extreme. Data on the frequency distribution of the individual COCSTOC measurements found in a natural population was subjected to a factor analysis which identified 18 occlusal patterns (combinations of traits) and five unique or independent traits or anomalies.

Forensic dentistry.

Data and information derived from the teeth and oral structures have been used successfully where other methods of identification or determination of criminal responsibility have failed. In many instances, the techniques used in forensic dentistry bring evidence to a point of certitude that would otherwise be lacking. Teeth, by their formation and structure and their relative permanence, record and retain the history of their development and the history of the incidents to which they have been subjected. Generally, the characteristics of formation and structure of teeth contribute information pertinent to probabilities of relationships and probabilities of events. As a result, these characteristics are highly useful in affirming or negating evidence as circumstantial or as factual.

Congenital absence of teeth: a review with emphasis on inheritance patterns.

Congenital absence of teeth is a heritably phenomenon probably most often passed to each generation by an autosomal dominant pattern with incomplete penetrance and variable expressivity. Correlation of hypodontia with systemic disease leads to the hypothesis that this frequent dental anomaly may in some cases be a microform of systemic ectodermal dysplasia.

Llama herd health.

Herd health programs for llama owners offer veterinarians the opportunity to provide preventive care to an expanding market. Some aspects of llama herd health programs are unique but should be based on herd health principles used for other livestock species. Vaccinations, parasite control, nutritional consultation and reproductive examinations, coupled with feet and teeth care, are some areas to cover in herd health programs. Each herd will have slightly different requirements and the program should be tailored to the goals of the producer.