104 ANNALS NEW YORK ACADEMY OF SCIENCES 



the upgrowing dentary without interfering with the other muscles. No 

 doubt its importance increased when the new joint was formed between 

 the dentary and the squamosal and when the old anterior pterygoid 

 muscle became reduced. Its subdivision into two slips followed, which 

 are the external and internal pterygoid of mammals. 



The digastric muscle of mammals (Fig. 4) represents a part of the 

 second constrictor of the Pisces, joined with one of the ventral muscles 

 of the throat region (A, V 3 ). It is a muscle with two bellies, a tendon 

 usually separating them, but this varies in different forms; so we may 

 say that the muscle varies from the so-called monogastric to the typical 

 double-bellied condition. As stated below, the muscle is constant in mam- 

 mals with the exception of monotremes and some edentates. It is the 

 only compound muscle in the muscles of the jaw and represents two 

 muscles, one innervated by the facialis, the other by the ramus mandibu- 

 laris trigemini, joined end to end, but still retaining the old innervation. 

 The older anatomists all homologized the posterior belly of the digastric 

 with the depressor mandibular of reptiles and amphibians, but the work 

 done on the innervation by Schulman, Lubosch, Euge and others has 

 shown that there is a common origin for them, but that they represent 

 different slips from the same constrictor. 



The variation of the digastric has been discussed by Chaine, Toldt, 

 Bijvoet, Parsons, Eouviere, Fiirbringer, Dobson, Futamura and others, 

 so that there is not much left to work out in this line, although the inter- 

 pretations of the authors are very variable. 



Chaine (1914) classifies the digastric of mammals as follows: 



(a) Those with two bellies : 





Macropus 



Brady pus 



Delphinus (Toldt) 



Artiodactyla 



Rodentia 



Carnivora 



Chiroptera 



Pteropus 



Insectivora 5 



Simiaa 



Prosimiae 





(b) Those with a single belly: 





Hydrochcerus 



Cavia 



Dolichotes 



Lepus 



(c) Digastric absent: 





Monotremes 



Delphinus 



Tatusia 



Tursiops 



5 In Talpa europcrns there is a small tendon from the posterior belly to the mandible. 



