ADAMS, PHYLOOENY OF THE JAW MUSCLES T05 



Parsons' views (1898, pp. 436-437) are expressed in the following 

 quotations : 



The most important point to bear in mind, in considering tliis muscle, is tliat 

 it is not always really a digastric, but that even when it appears monogastric 

 it has a double nerve supply. Our knowledge of the anatomy of fishes tells 

 us that the muscles developed from tlie first, or mandibular arch, are supplied 

 by the fifth nerve, while those formed in the second, or hyoid arch, derive 

 their nerves from the seventh. In three Ornithorhynchi I found a muscle 

 running from the sub-hyoid septum, outward and a little forward to the man- 

 diljle, not far from the angle ; it was supplied by the fifth nerve and lay super- 

 ficial to the mylohyoid muscle, of which it appeared to be a delamination, and 

 with which the fibers had the same general direction. In the same animal a 

 single muscle runs from the long, tubular external auditory meatus to the sub- 

 hyoidean septum ; it is supplied by the facial nerve, and probably corresponds 

 to the combined stylohyoid and posterior belly of the digastric. From this I 

 am inclined to tliink that the anterior belly is an older muscle than the pos- 

 terior, or, in other words, that the anterior belly is differentiated from the 

 mylohyoid layer before the posterior belly is split off from the stylohyoid. 

 This possibility may be the reason why the absence of the anterior belly of the 

 digastric is rare in man, but the absence of the stylohyoid is fairly common. 



Describing the digastric with two bellies, Parsons says: 



In the first [type] the anterior and the posterior bellies are separated by a 

 considerable length of tendon. The posterior belly runs forward until it is 

 over the hyoid and then runs inward and meets its fellow from the opposite 

 side, forming an arch. The anterior bellies of the digastric spring from this 

 arch and go forward. The muscle is not attached to the hyoid but is connected 

 by connective tissue. 



This type is found in many rodents (sciuromorphs, Pleromys), in 

 most cynomorph monkeys and in certain anthropoid apes (orangs, chim- 

 panzee). 



Parsons places under his second division those with a pseudo-mono- 

 gastric muscle. Here the muscle seems to have one belly, but there is 

 always a small rudiment of the median tendon. This type attaches to 

 the mandible, midway between the symphysis and the angle, and instead 

 of forming fiat planes meeting along the median line of the neck they 

 are rounded, as some expanse of mylohyoid separates them. 



Parsons' third type is that found in man, where the bellies are distinct 

 with a tendon separating them, the anterior bellies not meeting in the 

 median line of the neck but forward near the symphysis. This type is 

 found in most lemurs, many monkeys, especially the platyrhini, gibbons 

 (Hylohates) and in many specimens of gorilla. The stages are all united 

 bv transitional stages. 



