AD A1IS, PHYLOGEXY OF THE JAW MUSCLES 145 



could not have been carried through to the Mammalia, or at least there is 

 nothing to show that such was the case, as the new stapedial muscle is 

 regarded as a slip from the C 2 md or from the reptilian depressor man- 

 dibular The preoperculum, which is considered to be the squamosal of 

 the forms above the Pisces, retains its old connection with the temporal 

 muscle mass, so that the masseter (or superficial) slip of the urodeles and 

 anurans is still associated with the transformed preoperculum. In the 

 Amphibia the piscine operculum is absent, so that the three muscles 

 attached to it in the Pisces would be lost. 



The simplest condition of the musculature is found in the urodeles, 

 where the muscles are well separated, but are not so specialized as in the 

 Anura and retain more of their piscine condition. They are divided into 

 slips, three or four in number. These are separate slips that compare 

 with those indicated in the muscles of the reptiles, but are not regarded 

 as fully homologous with them. The superficial slip is the typical mas- 

 seter-like muscle with the same general direction of the fibers and the 

 same relation to the main mass. The deeper muscles of the peculiar 

 "Temporal mass" have a peculiar specialization, in that one slip extends 

 back over the skull and arises no longer on the parietal, but on the neck 

 vertebras, sometimes as far back as the fourth cervical vertebra. (See 

 urodeles.) The anterior temporal, which arises from the side of the skull, 

 behind the eyes, and runs outward and backward to the inner side of the 

 mandible, is apparently comparable to the pterygoideus anterior of rep- 

 tiles. This pterygoideus anterior has separated from the under side of 

 the capiti-mandibularis mass, but does not arise from the pterygoid. 



The Anura show the highest specialization found in the amphibians, 

 as the muscles are not only divided into slips, but the slips are well sepa- 

 rated, both in their origins and in their insertions on the mandible. The 

 striking thing in connection with the anuran jaw muscles is their con- 

 nection with the auditory region, for the muscles of mastication and some 

 of the neck muscles serve to keep the tympanum stretched taut. The 

 division of the capiti-mandibularis superficialis (masseter) is suggested 

 in the urodeles, where the fibers of the single muscle show an interme- 

 diate condition. The depressor mandibular is the same throughout the 

 group. • It has two slips, one from the skull and one from the dorsal 

 fascia. They represent again the C 2 md of Ruge, which is the mother 

 mass of this group innervated by the VII nerve. It seems from the 

 innervation, origin and insertion that the individual slips may be homolo- 

 gized throughout the Amphibia, and the homologies as they appear are 

 shown in Table II. 



