THE XER\ OUS SYSTEM 479 



and treat material by the Marchi or the Weigert method, according to the time 

 elapsed between the operation and the death of the animal. 



BRAIN 



General Structure 



The principal peculiarities of the brain as distinguished from 

 the cord depend upon two factors; certain peculiarities of the re- 

 ceptors and effectors of the head and the development of higher 

 coordinating apparatus in the central nervous system of the head. 



Besides the receptors of the general senses (p. 442), there are in the 

 head the highly specialized receptors of smell, sight, hearing and posi- 

 tion (semicircular canals), which are respectively concentrated into 

 certain localities and form, together with certain accessory structures, 

 the organs known as the nose, eye and ear. Each of these groups of 

 receptors has its own special connection with the brain (nerves I, II, 

 and VIII) and its own paths within the latter (see below). The 

 special receptors of taste show a less degree of aggregation into an 

 organ and, together with other visceral receptors, are innervated by 

 afferent portions of a group of nerves (VII, IX and X) which have a 

 common continuation within the brain. The remaining somatic 

 receptors of the general senses, scattered over the anterior part of the 

 head, are innervated by one nerve (V) having its own central con- 

 tinuations. It has already been stated that all the afferent periph- 

 eral neurones which innervate these receptors (except the mesen- 

 cephalic V) follow the general law of having their bodies located out- 

 side the neural tube. The central processes (root fibres) usually split 

 on entering the tube, but the descending arms are the longer. Nerves 

 I and II present certain special peculiarities. 



The splanchnic effectors of the head include the usual splanchnic 

 effectors — smooth muscle and glands — and also the branchial stri- 

 ated voluntary muscles. The somatic effectors are the remain- 

 ing (myotomic) striated voluntary muscles. The striated voluntary 

 muscles of the head fall into three groups; those of the eye (somatic); 

 of the jaw, face, pharynx and larynx (splanchnic, modified branchial 

 musculature); and of the tongue (somatic). The peripheral path 

 to the smooth muscles and glands follows the same general law as in 

 the body, i.e., neurone bodies in the central nervous system send pre- 

 ganglionic root fibres to sympathetic ganglion cells, -which in turn 

 send their axones to the eft'ectors (p. 445). The peripheral path to 



