ADA2I.S, PHYLOOENY OF THE JAW MUt^CLEH Kj; 



Parsons also mentions a fourth type, where there is a true nionogastric 

 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 Chaine'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 V3, 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 Ornitliorliyncliiis shows the primitive stage of the 



Description of Figure 2 



Variations of tlie digastric in mammals. (Mainly after Ch-iine and Parsons.) 



Tlie digastric muscle is found only iu t.vpical 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 

 V3. 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. 



Probnbly the most primitive type is seen in the monotreme OrnithorhyncJius (10). 

 Here the "detrahens mandibul» anterior" (2). 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 Mairopiis (.5) shows well the association of the anterior belly («) with 

 the mylohyoid and the connection of both bellies with the bisi-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 Xos. 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 anterfor 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 (2i 

 or to the posterior part. 



