68 THE BRAIN OF THE TIGER SALAMANDER 



physiological criterion marks also the most fundamental structural 

 analysis of the nervous system. Anatomically, the somatic systems 

 of peripheral organs and nerves and central adjustors, including the 

 proprioceptors, are, in general, rather sharply distinguished from the 

 visceral. The systems are cross-connected by internuclear tracts, and 

 some sensory systems, like the olfactory, may serve, on occasion, 

 either somatic or visceral adjustments. 



A phylogenetic survey of these systems reveals remarkable plastic- 

 ity in their interrelationships. Thus, taste buds, which in most verte- 

 brates are typical interoceptors, may in some fishes be spread over 

 the external surface of the body, where they serve exteroceptive func- 

 tions, with corresponding changes in the anatomical pattern of the 

 central apparatus of adjustment (chap, x; '446). On the motor side 

 the apparatus of feeding and respiration exhibits still more remark- 

 able transformations. In fishes this musculature is connected with the 

 visceral skeleton — jaws, hyoid, and gill arches — and the functions 

 performed are obviously visceral, though the larger part of this 

 musculature is striated. The related parts of the nervous system are 

 classified as special visceral motor. With suppression of the gills in 

 higher animals, some of these muscles undergo remarkable transfor- 

 mations. Those which are elaborated to form the mimetic facial 

 musculature of mammals become physiologically somatic ('22, '43). 



The classification of peripheral end-organs and their related nerves 

 which has proved most useful grew out of the analysis of these nerves 

 into their functional components, to which reference has just been 

 made, by histological methods. Serial sections through the entire 

 bodies of small vertebrates differentially stained for nerve fibers 

 enable the investigator to reconstruct not only the courses of the 

 nerves but also the arrangement of the functional systems repre- 

 sented in each of them and to follow these components to their 

 peripheral and central terminals, a result that cannot be achieved by 

 ever so skilful dissection. The first successful application of this 

 method was Strong's analysis of the nerve components of the tadpole 

 of the frog in 1895, a fundamental research which provided the gen- 

 eralized pattern which prevails, with endless modifications of details, 

 throughout the vertebrate series, as has been abundantly demon- 

 strated by numerous subsequent studies by many workers. 



This was followed in 1899 by my Doctor's dissertation on the nerve 

 components of the highly specialized teleost, Menidia. From these 



