THE MUSCULAR SYSTEM 245 



of the anatomical details; as they are not in accord with one 

 another, however, they cannot all be right, and the remarkable 

 degree of correspondence in bones and muscles, not merely in 

 the salamanders, but also in many mammals, including man, a 

 correspondence that is obvious and easily apparent, is a strong 

 argument in favor of a natural syntropic comparison, as given 

 here. The embryological history, moreover, so far as it is 

 given, shows no sign of such a torsion or reversal as is de- 

 manded by the antitropists, but presents as the first stage of the 

 fore and hind limb, two pairs of lateral flaps, each with a 

 cranial and a caudal border and a dorsal (outer) and a ventral 

 (inner) surface. Of these the cranial border becomes respec- 

 tively the radial and tibial side of the future limb, the caudal 

 border the ulnar and fibular. The muscles of the original 

 dorsal and ventral surfaces remain in their primary position 

 and may be compared in the two limbs; in the distal portion 

 the dorsal muscles become extensors, the ventral, flexors, in 

 each limb. The embryological history thus furnishes a definite 

 proof in favor of the hypothesis of syntropism, or that of direct 

 comparison, limb with limb, in the normal position, and this 

 theory is espoused at the present time by the majority of in- 

 vestigators. 



The visceral musculature differs from the axial-appendicular, 

 thus far considered, in its derivation from the ventral portions 

 of the mesoderm, that is, from the hypomeres instead of from 

 the epimeres. The skeletal parts w r ith which it is associated 

 are those of the visceral arches and their derivatives, including 

 the jaw and the hyoid, and, in the higher forms the numerous 

 cartilages of the larynx and the auditory ossicles. This system 

 has thus its most extensive though perhaps not its most special- 

 ized development among the fishes, for here the gill-arches are 

 functional and need to be regulated by systems of levators, 

 depressors, constrictors, dilatators and so on, which often attain 

 a high degree of complexity. In the amphibians, where, in 

 spite of the existence of gill-slits, at least in the larva, there is 

 tut little need of controlling the movements of the separate 

 arches so precisely, the visceral musculature appears in a 



