214 Basic Structure of Vertebrates 



which is generally regarded as a territory of visceral centers. But in 

 position in the body, in their histologic structure, and in being volun- 

 tary, they are of somatic nature. They and their nerves are commonly 

 called "visceral." But no mere calling of names can abolish the am- 

 biguity of their nature. They confront us with the alternatives of 

 asserting that vertebrates have some external "viscera," or else 

 admitting that some somatic muscle can come from unsegmented 

 mesoderm and that some somatic motor centers may occur at that 

 level of the brain which is mainly visceral. 



The existence of external taste-receptors in the skin of the body 

 of some fishes is similarly perplexing. Taste is generally regarded as a 

 visceral sense because its receptors are ordinarily in the mouth and 

 pharynx, and because it has to do with food. Taste being thus essen- 

 tially visceral, the external taste-buds, in spite of their position, are 

 commonly called "visceral" receptors, and a special "visceral-sen- 

 sory" branch of the facial nerve (VII) innervates them. The embryonic 

 origin of the external taste-buds is not certainly known. It is possible 

 that their sensory cells are derived from the lining of the mouth or 

 pharynx by migration of cells out via the mouth or pharyngeal clefts. 

 It is even uncertain whether the internal taste-buds are derived from 

 pharyngeal endoderm or from the infolded ectoderm that lines the 

 mouth. Whatever their origin, we may either assert that some fishes 

 have a visceral sense in the external skin, or else admit that, while 

 the receptors for taste are usually visceral, some vertebrates may 

 have accessory somatic gustatory receptors and that the facial nerve 

 may have a somatic special-sensory branch. 



The fact is that animals resort to such diverse expedients in the 

 achievement of their vital necessities that rigid distinctions between 

 what is "somatic" and what is "visceral" cannot be made to hold 

 for all the structures and functions of all animals. In some amphibians 

 respiration is carried on partly in the poorly developed lungs but 

 mainly through the thin epidermis over the entire body. Is the skin 

 therefore "visceral"? The muscles connecting adjacent ribs (inter- 

 costal muscles) of mammals are as important in relation to lung- 

 breathing as the branchiomeric muscles are in relation to gill-breath- 

 ing. Are the intercostal muscles "visceral"? 



Between striation of muscle-fibers and voluntary action, there 

 is no necessary correlation. The external transverse bands on the fiber 

 are incidental to a complex internal structure which gives the fiber a 

 high capacity for contractile efficiency. In any situation, whether 

 somatic or visceral, where muscular action of high efficiency is needed, 

 the muscle becomes striated. Voluntary control of muscular activity 



