.l/>.Ll/.s'. PHYLOOENY OF THE JAW MUSCLES 57 



It is import;iiit to notice that while the myotomes and the muscles derived 

 from them (such as the limb muscles) receive their motor nerves exclusively 

 from the ventral roots of the spinal nerves, the splanchnic muscles, the skin, 

 the mucous membrane and their sense organs are supplied by the dorsal, 

 ganglionated roots. 



The ombrvolog-y of Petromyzon, AmpJbioxus and the fishes give some 

 very illuminating evidence of the evolution of the muscular and the 

 nervous system in the early vertebrates, and from them we may get a better 

 understanding of the muscles and serial nerves of the higher vertebrates. 

 Thus a study of the embryology gives much aid in tracing the homology 

 of the capitimandibularis muscle in the reptiles, where it is an undiffer- 

 entiated mass, as in the embryo of man. i^gain the problem of the 

 musculature of the middle ear is very much clarified by the embryological 

 investigations of Gaupp, Versluys, Fuchs and other students of the ear, 

 where the development gives an explanation of the origin of these ele- 

 ments. 



As the nerves and muscles start out together very early in the life of 

 the embryo, the nerve supply has always been taken as one of the best 

 tests of homology. It is almost an axiom in comparative anatomy that a 

 muscle is always followed by its nerve, and while in most cases this is 

 true, it is occasionally not true. The stapedius muscle of the middle ear 

 illustrates the constancy of the nerve supply. This muscle is followed 

 from its position as a levator of the hyoid arch through many changes 

 to its final resting place in the middle ear, with the innervation by the 

 facialis remaining constant. The tensor tympani muscle also represents 

 a slip of the reptilian pterygo-mandibularis that has shifted to the middle 

 car. The facialis in its migration from the hyoid arch over the face gives 

 another fine example of the constancy of the nerve supply to the muscle. 

 In man the platysnia, sphincter colli and facial muscles of the eye, nose 

 and mouth have migrated from their original position on the side of the 

 neck to the most anterior portion of the face. In this case the seventh 

 nerve has overlapped the territory of the trigeminus and the serial nerves 

 anterior to it, while the muscles of the fifth nerve have remained in their 

 original position. The pectoralis and latissimus dorsi muscles of man 

 give another example of shifting muscles that carry their nerves with 

 them. They shift from their original position to cover large areas of the 

 trunk that were formerly innervated by the several serial nerves of the 

 myotomes. In the external oblique of man and the rectus abdominis 

 there is a fusion of the elements, their originally separate condition being 

 shown by their nerve supply. 



The digastric is also a muscle witli a double nerve supply, and is very 



