150 ANNALS NEW YORK ACADEMY OF SCIENCES 



upper part of the reptilian depressor mandibular as it has been shown in 

 the sections on birds and reptiles that the depressor is in the position to 

 gain the relations of the stapedial muscle of the mammals. It hardly 

 seems possible, from the conditions seen in Cynognathus, that the de- 

 pressor mandibular has been carried over from the reptiles as the posterior 

 belly of the mammalian digastric. The most satisfactory conclusion ap- 

 pears to be that a part of the depressor became the stapedial muscle and 

 that the mammalian digastric is a new slip from the second constrictor 

 mass, which was still retained in the reptiles (Ruge). Wilder (1909), 

 in discussing the origin of the stapedius, says : "A portion of the posterior 

 belly, that is, of the second levator, becomes separated from it in the rep- 

 tiles, and follows the stapes into the middle ear, whence it becomes the 

 stapedius muscle, innervated by a special branch of the facialis." That 

 the stapedius muscle is an ancient muscle appears to be ' shown by the 

 great separation of the digastric and stapedial nerves, both of which are 

 branches of nerve VII. In other groups of related muscles the nerves, 

 although branching and becoming more than mere nerve twigs, still indi- 

 cate their relationship by their proximity to each other, as in the case of 

 the subdivisions of the nerve V 3 , where the muscles that are closely related 

 have their nerves coming off close together. The stapedial nerve comes 

 off at the upper part of the facialis, while the digastric nerve comes off 

 far below it. This seems to help the hypothesis of the derivation of the 

 stapedial and the wide separation in time from the appearance of the 

 mammalian digastric. 



Two muscles, the tensor tympani and tensor palati, appear in the mam- 

 mals for the first time. They are innervated by the ramus mandibularis 

 of the trigeminus and, according to Gaupp, their homology may be looked 

 for in one of the pterygoid muscles of the reptiles, namely, the "pterygo- 

 mandibularis" of Bradley, which is the "anterior pterygoid" of the pres- 

 ent work. This muscle is in the position that would permit it to be 

 drawn into the middle ear as the tensor tympani, and it is also in a posi- 

 tion to give rise to the tensor palati. 



The anterior pterygoid of reptiles is attached to the posterior end of 

 the mandible, and during the change from the reptilian to the mamma- 

 lian condition it could be drawn into the middle ear without much change, 

 as has been shown above. A review of Gaupp's discussion, however, leaves 

 me in doubt whether these muscles have both come from the reptilian 

 pterygoideus anterior or from the reptilian external pterygoid or from 

 both. Their connection with the pterygoideus internus of man would 

 indicate their derivation from the deep portion of the capiti-mandibularis 

 of reptiles. The insertion of the tensor tympani on the handle of the 



