Johnston, Functional Divisions of Ncruous System. 89 



can be reduced (in lower vertebrates) to terms of these func- 

 tional divisions as they are represented in the spinal cord and 

 medulla oblongata. 



The several functional divisions are present in the most 

 complete form in the medulla and cranial nerves of fishes, and 

 the following definition of them is taken from this region. 

 A. Afferent (sensory) divisions. 



a. Somatic sensory: stimuli received from the external 



environment ; giv,e rise to reflexes (locomotor or 

 other) which directly affect (modify) the animal's 

 relation to its environment (in man, commonly give 

 rise to sensations and conscious reactions). 



1. General cutaneous sub-division ; brain centers con- 



tinuous with the dorsal horn of the cord and in part 

 highly specialized as the centers of (2) ; fibers con- 

 stituting components of the V, IX, and X roots 

 and reaching their final distribution by way of vari- 

 ous rami of the cranial nerves ; innervating the skin 

 without special sense organs (tree nerve endings). 



2. Acustico-lateral sub-division : brain centers primi- 



tively identical with those of (i) but in part more 

 highly specialized in all present vertebrates ; fibers 

 constituting the post- and pre-auditory lateral line 

 roots and the root of VIII ; innervating special 

 sense organs in the skin which are genetically re- 

 lated (pit and canal organs, Savi's vesicles, am- 

 pullae ofLoRENZiNi, and the ampullae of the internal 

 ear). 



b. Splanchnic sensory: stimuli received from the lining 



of the alimentary canal and irom special organs in 

 the branchial cavities, mouth, and on the surface of 

 the head and body ; give rise to reflexes which af- 

 fect the organic activities (nutrition, respiration, 

 circulation) and do not commonly produce sensa- 

 tions and voluntary movements. 

 I. General splanchnic sub-division : brain centers con- 

 tinuous with the region of Clarke's column of the 



