140 ANNALS NEW YORK ACADEMY OF SCIENCES 



This separation of the muscle slips from the mother mass may be shown 

 in embryology; for the muscle groups start as undifferentiated masses 

 and then become divided into the different slips. Lubosch (1913) de- 

 scribes the separation from the "temporal mass" of a slip which becomes 

 the "pterygoid" of the urodele. This splitting of the muscle is quite 

 definitely known, and in this paper I have made frequent use of this fact 

 in endeavoring to determine the homologies of the different groups and 

 in reconstructing the muscular systems in extinct forms. (See chapter 

 on reconstructions.) 



Some individual muscles may, however, be followed through all the 

 classes of vertebrates. If the history of a bone has been traced from the 

 Pisces to the Mammalia, there is no reason for assuming that the muscles 

 associated with it have changed, provided that they are present in all of 

 the classes and have retained their origin, insertion and to a certain ex- 

 tent their function. Perhaps the hyomandibular (= stapes), the pre- 

 opercular (= squamosal), and other bones whose history is pretty well 

 known, might be considered as having taken their muscles with them 

 throughout the evolutionary changes from the fishes to mammals, if there 

 is no mechanical or other reason for the dropping of the old and develop- 

 ment of new muscle slips. Muscles are pliable tissues having the power 

 of changing, either by shifting their origin or, if there is no demand for 

 their service, by dropping out. Vestigial muscles found in each of the 

 classes have been carried over from an earlier class and have lost their 

 usefulness and atrophied. A long list of such muscles might be com- 

 piled from the Mammalia or from any class. The vestigial muscles of 

 the ears in Homo, carried over from the simian stage, vestigial muscles 

 in birds, carried over from the Eeptilia, the vestiges of the levator arcus 

 palatini in Cryptobranchus and Amphiuma, carried over from the Pisces 

 (Lubosch, 1913, p. 71), the "adductor maxillae" in Cryptobranchus from 

 the Pisces, are all vestigial and more or less functionless muscles carried 

 over from one class to another. 



Homology of the Jaw Muscles in the Pisces 



(Table I) 



The primary division of the musculature may be made with the inner- 

 vation as a guide, as there is a natural grouping of the muscles of the 

 head into two systems — the muscles innervated by the fifth or trigeminus 

 and those innervated by the seventh or facialis nerve. The muscles also 

 divide into the same two divisions if we group them first as muscles an- 

 terior to the quadrate region and secondly as those posterior to it. The 



