110 ANNALS NEW YORK ACADEMY OF SCIENCES 



antl all may be more or less right. The double innervation and the 

 tendon seem to make certain the fact that the muscle has a double origin. 

 Some of the older writers speak of a single-bellied digastric, but most 

 of them agree that some evidence of the double origin always exists, and 

 that in the monogastric muscles remains of the tendon are in evidence in 

 the middle of the muscle, or, as in Lepus, the anterior muscle may be 

 almost gone while the tendon remains to represent the junction of the 

 two parts. In a few forms undoubtedly one of the muscles is missing — 

 that is, one belly is missing and it is not really a monogastric muscle but 

 a single belly in the sense that either the anterior or the posterior belly 

 has disappeared, and not that the one represents both bellies. 



So far I have spent very little time on the mylohyoid and the other 

 ventral muscles, reserving them for future work on the comparative 

 musculature throughout the vertebrates. The discussion of this phase 

 of the jaw muscles is made very brief and merely points out some of the 

 comprehensive work of Toldt, Chaine, Rouviere, Bijvoet and others on 

 the digastric musculature in the mammals. 



The relation of the jaw muscles to the special types of dentition is very 

 close throughout the mammalian groups. In fact a highly differentiated 

 muscle system in which each muscle does a certain type of work alone 

 makes possible the highly specialized dentitions of rodents and ungulates. 

 The teeth of mammals are, roughly, of three kinds — herbivorous, carniv- 

 orous and degenerate. The herbivorous dentition is correlated with the 

 peculiar type of condyle and glenoid joint that makes its specialized work 

 possible. The condyle of the typical herbivorous animal is much rounded 

 and the articulation in the glenoid cavity is flat, so that there is much 

 freedom of movement. In chewing the jaw is rotated in a lateral, antero- 

 posterior and vertical movement, so that to accommodate this movement 

 the articulation must be very loose. Accordingly, there is always a Avell 

 developed pterygoid region in the basicranial region, with well developed 

 pterygoid muscles. This strong development of the pterygoid muscles is 

 characteristic of Macropus, Halmaturus, Castor, Sciurus, Equus and Bos. 

 In these animals the molar teeth bear cross-ridges and the transverse mo- 

 tion needed for trituration is given by the strong pterygoid muscles. 

 Many herbivorous mammals chew on one side at a time. The pterygoid 

 muscle, acting in connection with the large temporal and masseter 

 muscles of one side, make this movement possible. 



In the rodents where the symphysis is weak the pterygoid muscles also 

 counterbalance the pull of the huge masseters, as in some of the forms 

 with a weak symphysis the pull of the masseters alone would tear the 

 jaws apart. The carnivorous mammals have an entirely different ar- 



