o' > - c-> II B RARY OF THE UN IVE.R.SITY OF ILLINOIS 590.5 FT v. 28-29 The person charging this material is re- sponsible for its return to the library from which it was withdrawn on or before the Latest Date stamped below. Theft, mutilation, and underlining of books are reasons for disciplinary action and may result in dismissal from the University. UNIVERSITY OF ILLINOIS LIBRARY AT URBANA CHAMPAIGN L161 O-1096 THE LIBRARY Of THE ZOOLOGICAL SERIES APR 1 3 1944 OF UNIVERSITY Of ILLINOIS FIELD MUSEUM OF NATURAL HISTORY Volume 29 CHICAGO, MARCH 28, 1944 No. 10 MASTICATORY APPARATUS OF THE SLOTHS BY HARRY SICKER FOUNDATION FOB DENTAL RESEARCH, CHICAGO COLLEGE OF DENTAL SURGERY A descending processus of the zygomatic arch has been developed independently in widely separated orders of mammals. It is charac- teristic of the living Bradypodidae and of many extinct edentates (Glyptodontidae and Gravigrada). It is found in Diprotodon and indicated in the Macropodidae. In the Entelodontidae it reached its highest development. Although several explanations have been advanced for its functional significance in the entelodonts no attempt has been made to correlate its presence in the Bradypodidae to a specific functional adaptation of the masticatory apparatus. The present investigations disclose the curious fact that the mechanism of mastication in the two-toed sloth is, in a sense, of an opposite type from that in the three- toed sloth. The present study is based on the skeletal material of Field Museum of Natural History and the dissection of one specimen of Bradypus and one specimen of Choloepus from the United States National Museum. I wish to express my appreciation to these institutions for their generosity. The drawings are the work of Miss Peggy Collings. REVIEW OF LITERATURE Schulman (1906) gave a fairly accurate description of the mas- seter muscle in Choloepus. He states that "the masseter muscle of Ch. Hoffmani arises from both surfaces of the zygomatic arch, which is partly represented by a ligament, and mostly from the zygomatic bone; it does not split completely into an outer and inner layer according to its outer and inner origin. On the contrary, the mas- seter remains united immediately below the zygomatic arch." . . . "The main direction of the fibers is from anterior and superior to posterior and inferior." . . . "The masseter is a multipennate No. 557 161 ttOFULUB 162 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. 29 muscle, the entire mass of which converges toward the angulus mandibulae." . . . "The strong tendency of the fibers to assume a position in the sagittal plane leads to a combined pressure of the mandible anteriorly and superiorly against the maxilla." Lubosch (1908) investigated the temporo-mandibular articula- tion in Choloepus and Bradypus. A disk is lacking in both genera. In correct occlusion of the teeth in Bradypus the condyle occupies only the most ventral part of the wide articular fossa. Statements as to the presence or absence of a sterno-mandibular (sterno-maxillary) muscle vary considerably. According to Windle FIG. 23. Superficial muscles of mastication of Choloepus. and Parsons (1889) it is absent in the Bradypodidae. Leche (1896) describes the sterno-mandibular muscle in Choloepus and remarks on its absence in Bradypus, according to Macalister. The descriptions of Edgeworth (1935) and Toldt (1908) of the sterno-mandibularis and its development agree in all points. Edge- worth states: "A sterno-hyoideus superficialis is separated from the Rectus cervicis. Its anterior end becomes attached to the trans- verse aponeurosis of the sterno-hyoidei and so with the digastricus anterior and a sterno-mandibularis is formed. This primary con- dition is present in Bradypus and Tolypeutes. It is almost certainly derived from a simple digastric muscle such as is present in Mar- supialia. It can be described as a digastric muscle to the transverse aponeurosis of which a sterno-hyoideus superficialis is attached, or a r c* WAT. f-/iST. 1944 MASTICATORY APPARATUS SICKER 163 sterno-mandibularis to the intersection in which the interhyoideus is attached." Observations of mandibular movements are not recorded in any detail. Beebe (1926) states that he could never see a lateral move- ment in a Bradypus kept for several months as a pet. OBSERVATIONS The differences in the masticatory region of the skull between Choloepus and Bradypus can be summarized as follows: Choloepus Bradypus Mandibular body long, ramus short, Mandibular body short, ramus long, angular process weak; condyle in angular process strong; condyle high occlusal plane of molars, long and oval; above pcclusal plane of molars, circular, condylar axes oblique, converging pos- knob-like; articulating surface regu- teriorly; articulating surface convex in larly convex; rough projection on the medial one-third, concave in the antero-medial circumference for attach- lateral two-thirds. ment of external pterygoid muscle. Descending zygomatic process tri- Descending zygomatic process nar- angular, short, vertical. row, long, oblique, directed ventrad and posteriorly. Pterygoid plate narrow, short, pos- Pterygoid plate wide, long, solid, terior part pneumatized. Articulating surface on temporal Articulating surface on temporal bone narrow in antero-posterior, wide bone wide in antero-posterior direction, in medio-lateral direction. 3 In the position of rest, the mandi- In the position of rest, the mandi- .^bular condyle in contact with the bular condyle in contact with the articulating fossa excepting a narrow anterior part of the articulating fossa "^ anterior strip. only, wide posterior area free, pi The muscles of mastication show only insignificant differences in Choloepus and Bradypus (figs. 23 and 24). Masseter muscle. Its origin is restricted to the zygomatic bone. The fibers arise from the anterior and posterior borders and the tip of the descending process and from the lower border of the temporal process. It is inserted to the lower border of the mandible to and > including the angular process, and to the outer surface of the ramus. ^ Here the insertion area is extended upward to the mandibular neck. ^ The fibers arising from anterior border and tip of descending process ^course vertically to the mandible, whereas the fibers of the posterior * part of the muscle are horizontal. A division into a superficial and \ a deep layer is arbitrary and incomplete. K> Zygomatico-mandibular muscle. Origin from zygomatic process of temporal bone and zygomatic ligament, insertion to the outer surface of coronoid process and adjacent area of ramus. Its fibers are directed ventrad and anteriorly. U. OF ILL. LIB, 164 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. 29 Temporal and pterygoid muscles are weak. The first shows no peculiarities. The pterygoids arise on the outer surface of the ptery- goid plate. The internal pterygoid inserts to the angular process, the external to the mandibular neck. In the temporo-mandibular articulation a disk is missing. The fibrous covering of the condyle is very thick. The capsule is loose. A sterno-mandibular muscle is present in Choloepus. The super- ficial fibers of the sterno-hyoid muscle have lost their attachment to the hyoid bone and are fused by a tendinous inscription to the FIG. 24. Superficial muscles of mastication of Bradypus. anterior belly of the digastric muscle. The posterior digastric ends in this tendinous inscription. In Bradypus a sterno-mandibularis is absent. The insertion of the sterno-hyoid muscle to the hyoid coincides with the attachment of the digastric tendon to the same bone. Dentition. The smooth glossy attrition facets of the tusk-like first teeth in Choloepus are not in contact in the rest position of the jaws. On the "molars" two types of attrition marks can be distin- guished: (1) Dull, irregular facets, giving the tooth irregular, pointed "cusps"; (2) glossy narrow facets that originate by sharp contact of the teeth during the masticatory stroke. They are found on the anterior (mesial) edges of the lower and the posterior (distal) edges of the upper teeth (fig. 25A). In Bradypus the first upper tooth is markedly reduced, the first lower tooth chisel-shaped. The larger attrition facet on the lower first tooth is found on its posterior (distal) surface. The glossy 1944 MASTICATORY APPARATUS SICKER 165 narrow attrition facets on the "molars" show a reversed arrange- ment compared with that of Choloepus. They are seen on the pos- terior (distal) edges of the lower and on the anterior (mesial) edges of the upper teeth (fig. 25B). DISCUSSION The development of a descending zygomatic process in the Bradypodidae is correlated to the change in the fiber direction of the masseter muscle. A considerable part of this muscle assumes a B FIG. 25. Lower jaw and teeth of A, Choloepus; B, Bradypus. horizontal course from the descending zygomatic process to the upper parts of the mandibular ramus. This portion of the masseter muscle functions as a strong protractor of the mandible. Its action, how- ever, is integrated into the masticatory movement in a strikingly different way in Choloepus and Bradypus. Although both are phyllo- phagous, the differences in the development of their anteriormost teeth would indicate a difference in their mode of cutting leaves. Unfortunately, detailed knowledge in this respect is still lacking. In spite of the similarity in the anatomy of the masticatory muscles, the differences in the structure of the mandibular articulation indi- cate different masticatory movements. A study of the attrition facets gives the final clue to the direction of the masticatory stroke in Choloepus and Bradypus. A sterno-mandibular muscle as a strong 166 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. 29 retractor of the mandible has developed in Choloepus and is lacking in Bradypus, although Bradypus clearly shows an initial stage in the formation of this muscle. The presence or absence of a sterno- mandibularis fits perfectly in the analysis of the masticatory move- ment of the Bradypodidae. The development of a sterno-mandi- bularis by a fusion of the superficial fibers of the sterno-hyoid to the anterior digastric adds considerable force to the retracting component of the latter. In Choloepus a function of the shear-like anterior teeth is possible only if the mandible is forcefully protracted during its closing move- ment. At rest and in a pure hinge movement these teeth are not in contact. When the anterior teeth have finished their cutting func- tion the mandible is retracted with considerable force and it is in this last phase of the closing movement that the upper and lower molars glide on each other and accomplish a grinding movement necessary to masticate the food. Proof for this movement is the location of the glossy attrition facets on the posterior edge of the upper and the anterior edge of the lower molars. The masticatory movement of Choloepus can be divided into three phases: (1) Opening movement: a hinge movement and the result of the contraction of the suprahyoids. (2) Cutting phase of closing movement: a combination of hinge movement and protraction of the mandible, the result of the con- traction of the masticatory muscles proper; the forward component is contributed by the external pterygoid and, mainly, by the hori- zontal portion of the masseter muscle. (3) Grinding phase of closing movement: a gliding movement, retraction of the mandible under pressure; the retraction is the function of the sterno-mandibular muscle, the pressure is provided by the vertical fibers of the masseter, temporal and internal ptery- goid muscles. In Bradypus the peculiar tusk-like development of the anterior teeth is lacking. They are chisel-shaped, the upper being rather small. The fact that the mandibular condyle is, in the position of rest, in contact with the anterior part of the glenoid fossa only, indicates that a posterior gliding of the mandible plays an important role in the masticatory movement. The masticatory stroke is directed from behind forward, as proved by the location of the glossy attrition facets on the molars. Their position is the reverse of that 1944 MASTICATORY APPARATUS SICKER 167 in Choloepus; they are found on the anterior edge of the upper and on the posterior edge of the lower teeth. The masticatory movement of Bradypus exhibits two phases: (1) Opening movement: a combination of hinge movement and retraction, the result of the contraction of the suprahyoids; the posterior gliding component is contributed by the digastric muscle acting without great force. (2) Closing movement: the grinding masticatory stroke, a combination of hinge movement and protraction of the mandible, is the result of the contraction of the masticatory muscles; the anterior gliding component is contributed by the external pterygoid and, mainly, by the horizontal portion of the masseter muscle. SUMMARY (1) The development of a descending zygomatic process in Choloepus and Bradypus is correlated with the development of a strong horizontal portion of the masseter muscle in both. (2) The insertion of the horizontal bundles of the masseter is extended high up to the region of the mandibular neck. (3) The protracting force of the horizontal fibers of the masse- ter, synergistic to the external pterygoid muscle, plays a different role in the masticatory movement in Choloepus and Bradypus. (4) In Choloepus the horizontal part of the masseter assures contact between the cutting "canines" during the first closing phase. In the second, grinding phase of the closing movement the mandible is forcefully retracted by the sterno-mandibular muscle. (5) In Bradypus the mandible is retracted during the opening phase by the digastric muscle, the condyle gliding into the posterior part of the wide glenoid fossa. At the end of the closing phase the horizontal part of the masseter effects the grinding movement by protracting the mandible with great force. (6) The presence of glossy attrition facets on opposite edges of the molars in Choloepus and Bradypus proves that the grinding movement occurs in opposite directions in these two animals. REFERENCES BEEBE, WILLIAM 1926. The Three-Toed Sloth, Bradypus Cuculliger Cuculliger (Wagler). Zoologica, 7, pp. 1-67. EDGEWORTH, F. H. 1935. The Cranial Muscles of Vertebrates. Cambridge University Press. 168 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. 29 LECHE, WILHELM 1874-1900. Muskulatur. Bronn's Klassen und Ordnungen des Tierreiches. Mammalia, 1, pp. 649-919. LUBOSCH, WILHELM 1908. Das Kiefergelenk der Edentaten und Marsupialier. Denksch. med. naturw. Ges. Jena, 7, 519-555. SCHULMAN, HJALMAR 1906. Vergleichende Untersuchungen iiber die Trigeminusmuskulatur der Monotremen. Denksch. med. naturw. Ges. Jena, 6, pp. 297-400. TOLDT, C. VON 1908. Der vordere Bauch des M. digastricus mandibulae und seine Varietaten beim Menschen. II. Sitz. Akad. Wiss. Wien, (3), 117, pp. 229-321. WINDLE, B. C. A., and PARSONS, F. G. 1899. On the Myology of the Edentata. Proc. Zool. Soc. Lond., 1899, pp. 314-339, 990-1017. A