ADAMS, PHYLOGENY OF THE JAW MUSCLES 139 



between these muscular systems and their general position has been the 

 same throughout. The trigeminus innervates the muscles of mastication 

 that lie in front of the quadrate, while the facialis innervates the muscles 

 behind the quadrate and those of the hyoid region. These regions in the 

 mammals overlap on the surface, for the reptilian sphincter colli has 

 crept forward over the deeper muscles of the trigeminus and has formed 

 the mimetic muscles of the face, so that while the deep muscles, anterior 

 to the hyoid arch, are innervated by the trigeminus the superficial muscles 

 are innervated by the facialis. This is an example of the faithfulness 

 with which the nerves follow the muscles in their migration (Ruge, 

 1897). 



Starting with the elasmobranchs, we find the adductor mandibular 

 muscles, which are undifferentiated masses, derived from the pro-branchial 

 muscles before the anterior branchial arches were transformed into jaws. 

 The adductors are not separated into special muscles, but represent the 

 "mother mass" of the chief jaw-muscles of the V 3 group in all the higher 

 classes of vertebrates. A depressor mandibular is not differentiated in 

 the elasmobranchs or in the other classes of Pisces, but is represented by 

 the second dorsal superficial constrictor throughout this group, a true 

 depressor first appearing in the Amphibia. 



In the development of the Amphibia from some of the Pisces the jaw 

 musculature was carried over in its general plan, but the muscle masses 

 took on the peculiar modifications needed in each group. We may con- 

 ceive that these "mother masses" C t and C 2 became differentiated in many 

 ways throughout the different classes of vertebrates, and that various slips 

 were given off from these masses which served their purpose and the needs 

 of the animal, only to be dropped in the future development, while new 

 slips arose in the higher forms. We may further conceive that these 

 mother masses were innervated by the nerves V 3 and VII, and that in 

 the first stages of the origin of a new muscle slip there was a mere branch- 

 ing- of the nerve into small twigs; in this stage there was no differentiation 

 into special nerves and no division of the muscles into separate slips, but 

 merely partially separated portions with the nerve twigs following and 

 gradually becoming more differentiated, as conditions demanded further 

 separation. Finally, when these slips were separated off from the main 

 mass, the nerve twig became a branch, and the muscle, having lost all 

 connection with the fibers of the parent mass, might be called a separate 

 muscle. This is my conception of the origin of the special muscles in the 

 different classes, and with this view the precise homology, except within 

 the class, is sometimes doubtful, unless very primitive or annectent forms 

 between widely separate groups are available for study. 



