Integrative Systems 213 



The afferent impulses transmitted from these proprioceptors do 

 not necessarily result in sensations which rise into consciousness. Many 

 of them are concerned with such automatic reflex mechanisms as that 

 of equilibration. 



SOMATIC AND VISCERAL ANOMALIES 



In regard to the distinction between "somatic" and "visceral" 

 structures, as discussed on pages 154-157, there are some things whose 

 status is either exceptional or ambiguous. 



The heart is a visceral organ but its muscles are striated, yet 

 they are involuntary. Its muscular walls, especially those of the 

 ventricle, are thick and contract strongly and quickly. The constituent 

 striated fibers of the muscle are, in various details of their structure, 

 quite unlike somatic striated fibers. Cardiac muscle arises from unseg- 

 mented mesoderm of the same sort as that which produces ordinary 

 nonstriated muscle. The beating of the heart is not only involuntary 

 but also automatic to the extent that it will continue, for a limited 

 time, after all nervous connections of the heart have been cut. 



Muscle which is striated, but involuntary, occurs in the terminal 

 regions, especially the anterior, of the wall of the digestive tube. In 

 mammals striated muscle continues from the pharynx back into the 

 esophagus, and, in the rabbit, even throughout its entire length. The 

 embryonic origin of these striated esophageal muscles is like that of 

 nonstriated muscles. Although striated, their action is involuntary. 

 At least in man, swallowing is under voluntary control until food 

 arrives in the rear part of the pharynx, but, once it has entered the 

 esophagus, no exercise of "will power" can prevent its going down. 



The diaphragm, dividing the coelom, may seem to lie in visceral 

 territory, and, as a muscular organ, it is concerned with the visceral 

 function of breathing. But its muscles are developed by growth of 

 mesoderm downward and backward from myotomes (usually three 

 pairs) in the cervical region of the embryo. The phrenic nerves, 

 supplying the diaphragm, are the third, fourth, and fifth pairs of 

 cervical nerves. The muscle-fibers of the diaphragm are of the typical 

 somatic striated sort. Its action is potentially voluntary, although 

 ordinarily it is automatically reflex. So far as its muscle is concerned, 

 the diaphragm is an ingrowing derivative of the body-wall and is 

 typically somatic in its segmental origin, in its structure, and in being 

 voluntary. 



The branchiomeric muscles are especially perplexing. In em- 

 bryonic origin they resemble visceral muscle (see p. 154) and primi- 

 tively they are concerned with alimentation and respiration. Their 

 nerves come from motor nuclei situated at that level of the medulla 



