ADAMS, PHYLOGENY OF THE JAW MUSCLES 151 



malleus would, on the other hand, indicate relationship with the anterior 

 pterygoid, which in reptiles is inserted on the back part of the mandible. 

 The topographic relations of the tensor palati suggest one of the cranio- 

 ptervgoid muscles of reptiles. The tensor veli palatini (tensor palati) is 

 called by Gaupp the homolog of the reptilian "pterygo-mandibularis" 

 (anterior pterygoid). He points out that Kostanecki (1891) in Didel- 

 phys showed the close relation of tensor tympani and tensor veli palati. 

 Killian (1890) also found that the two muscles were joined in the early 

 stages of apes, man, Cheiroptera, etc. In man the two muscles appear at 

 about the fourth month to become separate from the pterygoideus inter- 

 nus. These facts in the embryology would appear to show that tensor 

 tympani and tensor veli palati are phylogenetically related ; that they 

 come from the pterygoid musculature of the reptiles, probably from the 

 anterior pterygoid. Their close relations with the mammalian ptery- 

 goideus internus also indicates that they arose from the same region of 

 the temporal mass. This derivation of the tensor tympani from one of 

 the pterygoid muscles seems very plain, for it extends down along the 

 side of the eustachian tube in the mammals and is attached to the handle 

 of the malleus, so that the essential relations are not changed in the 

 transformation of a jaw muscle into an auditory muscle. 



The mammalian pterygoid muscles may possibly botli be derived from 

 the deep part of the capiti-mandibularis mass of reptiles — that is, from 

 the capiti-mandiV)ularis profundus or pterygoideus externus. The changes 

 in the posterior part of the jaw and in the skull would make it almost 

 impossible to derive these from the anterior pterygoid muscles. The diffi- 

 culty in deriving the pterygoid muscles of mammals from the anterior 

 pterygoid muscles of reptiles is that the latter are inserted on the posterior 

 part of the mandible, an element which became vestigial, while the former 

 are inserted wholly on the dentary. A study of tlie jaws and skull of 

 Cynognathus and the other cynodonts indicates that it would be difficult 

 to conceive clearly the transference of the insertion point of the anterior 

 pterygoid from the angular and prearticular region to the back part of 

 the dentary. On the other hand, the pterygoideus or capiti-mandiljularis 

 profundus, which is inserted in the region of the coronoid, is in such a 

 position that the upgrowing, ascending ramus of the dentary might well 

 invade its insertion area. In this way a muscular connection between the 

 wall of the cranium and the inner side of the dentary might easily be 

 established. As the reptilian palate and lower jaw became completely 

 transformed, it is to l)e expected that the characteristically reptilian 

 pterygoideus anterior would degenerate along with the elements on which 

 it was inserted. From the innervation of the mammalian pterygoid 



