THE MUSCULAR SYSTEM 



193 



muscles. There, will be noticed this difference in the proximal 

 muscles of the anterior and posterior limbs, that while those 

 of the latter arise almost exclusively from the girdle itself, 

 those of the anterior limbs are spread out fan-like over the 

 axial myotomes, and arise either from the myocommata or 

 from the integument. In the higher forms this difference re- 

 ceives much greater emphasis, and in both' birds and mammals 

 the appendicular muscles of the fore limbs completely enwrap 

 the body both dorsally and ventrally, thus concealing the axial 

 muscles entirely, from the hips to the neck, except in the ven- 

 tral abdominal region. 



Corresponding to the extensive development of the visceral 

 skeleton in the animal under consideration, the visceral muscu- 

 lature is also large and well shown. This occupies the ven- 

 tral and lateral regions of the head and neck, and includes the 

 massive muscles of the mandible, which extend over the top 

 and sides of the skull. In the higher forms these muscles, 

 with the exception of those of the mandible, lose in bulk, but 

 perhaps gain in complexity, supplying tongue, pharynx, and 

 the laryngeal region. 



Although thus far the facts presented rest upon a secure 

 morphological basis, and although it is comparatively easy to 

 follow the fate of the primary muscle masses as a whole in 

 the separate vertebrate Classes, the further emphasis of the 

 separate units which differentiate from the primary masses, 

 that is to say, the homology of the individual muscles, is a sub- 

 ject fraught with especial difficulties, and is one in which the 

 ground is still uncertain, owing to the lack of fixed principles 

 to direct the investigation. To begin with the axial muscles 

 in their undifferentiated condition as a series of similar myo- 

 tomes and follow out the various fate of their derivatives 

 throughout vertebrates as the parts become modified to sub- 

 serve countless special uses ; to start with the limb muscles in 

 the form of myotomic buds and follow the transformation 

 outwardly expressed by the varied shape of fin, wing or leg; 

 and finally to discover a fundamental plan in the complicated 

 visceral muscular system of selachians, and carry out the 



