ADAMS, PHYLOGENY OF THE JAW MUSCLES 107 



P arsons also mentions a fourth type, where there is a true monogastric 

 muscle in which the anterior or the posterior belly is suppressed. It is 

 found in lagomorph, or hare-like rodents. The posterior belly is repre- 

 sented by a small tendon in these. In many orangs the anterior belly 

 is wanting and the posterior belly is attached close to the angle. 



Parsons' division differs from Chatne's in that he takes into considera- 

 tion the condition of the anterior belly, whether joined with its fellow 

 of the opposite side or free from it. Humphrey considered that the 

 anterior belly was from the same myotome as the pterygoids, and the 

 posterior belly was from the hyoid arch muscles, or hyoid myotome. He 

 considered the tendon as a remnant of the myocommata connecting the 

 two myotomes. These divisions of the digastric as given by Chaine and 

 Parsons show some of the variations that are met with in this muscle. 

 The posterior part of the muscle is not troublesome, but the anterior part, 

 or the part innervated by nerve V 3 , has caused the anatomists much 

 trouble. 



Some of the ideas as to the homology of the anterior belly are interest- 

 ing, and show the diversity of opinion on this topic. Bijvoet (1908) gives 

 an interesting summary of the ideas of different authors as to its origin. 

 He considers that Ornithorhynchus shows the primitive stage of the 



Description of Figure 2 



Variations of the digastric in mammals. (Mainly after Chaine and Parsons.) 



The digastric muscle is found only in typical mammals and is not clearly recognizable 

 in the monotremes. It is a compound muscle typically consisting of anterior and pos- 

 terior bellies united by ligament. The anterior belly is probably a derivative of the 

 primitive throat muscles of reptiles and is innervated by the mylohyoid branch of nerve 

 V 3 . The posterior belly probably represents a separate slip from the stylohyoid muscle, 

 and both muscles are innervated by closely associated branches of the seventh nerve. 

 Probably both bellies of the digastric formerly converged toward the lower surface of 

 the basi-hyal cartilages, along with the mylohoid and other muscles, and the tendinous 

 portion between the anterior and posterior bellies may represent part of the fascia into 

 which they were formerly inserted. 



Probably the most primitive type is seen in the monotreme Ornithorhynchus (10). 

 Here the "detrahens mandibular anterior" (D. m. a.) (which may be a slip of the mylo- 

 hyoid) may represent the anterior belly of the digastric, while the posterior belly is not 

 yet separate from the stylohyoid (Parsons). 



The marsupial Macropus (5) shows well the association of the anterior belly (a) with 

 the mylohyoid and the connection of both bellies with the basi-hyal. In most types the 

 anterior and posterior bellies are both present and separated by tendon (as in Figs. 

 2, 4, 5, 6, 8, 9, 11). Either one of the bellies may be vestigial or wanting. In No. 7 

 the posterior belly is reduced to a small tendon. In No. 13 the anterior belly is tendi- 

 nous. The so-called monogastric types (3, 6, 9) generally show at least a vestige of the 

 tendinous septum between the two bellies. In Nos. 1, 12 the digastric is entirely absent, 

 being functionally replaced by other muscles. The posterior belly usually arises from 

 the paroccipital process of the exoccipital. The insertion of the anterior belly varies 

 greatly as well as its relations both to its fellow of the opposite side and to the mylo- 

 hyoid (cf. Nos. 2, 3. 7, 9, 11). It is primitively inserted on the inferior border of the 

 mandible beneath the masseter, but may shift either to the region of the symphysis (2> 

 or to the posterior part. 



A 



