ADAMS, PHYLOGENY OF THE JAW MUSCLES 109 



the m. jugularis transversus, the same mass from which the stylohyoid 

 is separated. He considers the digastric to be the result of the joining of 

 these two muscles. 



Toldt (1908) gives the following points on the anterior belly of the 

 digastric : 



( 1 ) The digastric as such is found only in the mammals. The attempts 

 to derive it from the muscles of the non-mammalia have not been success- 

 ful. The hinder belly is not the depressor mandibulge of the Sauropsida 

 nor is it derived from the depressor of the monotremes. (Schulman 

 shows that it is not derived from the depressor of monotremes.) 



(2) The depressor of the monotremes is a special primitive condition 

 and is a slip from the masseter innervated by V 3 . The writer thinks this 

 is not a primitive condition but a very specialized condition in a primitive 

 form. 



(3) The anterior belly of the digastric has its origin with the m. 

 mylohyoideus muscle and belongs with the visceral muscles that extend 

 along the floor of the mouth. They are innervated by Y 3 . 



(4) The posterior belly is a part of the stylohyoideus in the lower 

 vertebrates and is originally a visceral muscle intercalated between the 

 musculature of the mouth floor and throat and directly or indirectly con- 

 nected with the tongue bones. 



(5) The joining of this originally separate muscle to the digastric, as 

 well as the identity of the hinder belly and the stylohyoideus, is shown 

 well in the monotremes. The line of separation is shown by the inscriptio 

 tendinea. 



Toldt correlates the condition of the digastric with the type of food 

 that the animal eats and with the question whether the food is held in 

 the mouth or swallowed immediately. This, he thinks, gives an explana- 

 tion of the joining of the forward bellies in some forms and separation 

 of them in some of the carnivorous forms. But a comparison of the 

 varying forms of digastric seems to show that there is no correlation with 

 food habits, as there seems to be no definite functional criteria by which 

 the different conditions may be classified. 



From a review of the literature on the digastric the variations are 

 plainly evident both in the muscle and in the ideas concerning it. There 

 seems to be no very stable insertion for the anterior belly. Its insertion 

 varies from the inside of the chin, as in Homo, to the posterior part of 

 the mandible, on the angle in some other forms. Only the anterior part 

 is of uncertain origin, as most of the writers agree on the derivation of 

 the posterior belly. The anterior part lends itself to hypotheses, as 

 nothing definite seems to have been brought forward at the present time 



