114 



GENERAL ERIXCIPLES OF ZOOLOGY 



The Tubular System is fouiul only in Ihe chordatcs (lig. 79). The 

 vertebrate brain and spinal cord form a tuljc with greatly thickened walls. 

 In the centre lies the extremely narrow central canal, which wiiiens 

 anteriorly into the ventricles of the brain. In a transverse section the 

 nervous elements are seen grouped arountl the central canal in a manner 

 almost the reverse of that of the ganglionic type. On tlie periphery lies 

 a layer of nerve-tibres (the icliilc mailer) ; next is a central portion formed 



of ganglion-cclls and nerve-hbres 

 (the gray matter), \\\\\d\ is marked 

 off from the central canal b_v a 

 special epithelium (cpciidyma). In 

 addition there are modified sup- 

 porting cells which form a frame- 

 work {glia, neuroglia) for the 

 nervous parts. 



Relations of Nervous System 

 and Skin. — It has been ascer- 

 tained in almost all animals that 

 ///(■ )!ervenis svsle}ii arises jrom tlie 

 cetoderin. Therefore, in many 

 animals, the nerve-cords and the 

 ganglionic masses lie permanently 

 in the skin; in others only during 

 the development, later becoming 

 separated by splitting of! or by 

 infolding, and thus coming to lie 

 in the deeper layers of the hoAy (fig. 9). In the vertebrates and some 

 other higher animals, besides the body nervous system, there is a sympa- 

 ihetie system for the control of the vegetative organs which are not 

 intluenced Ijy the will. 



III. Sensory Organs. 

 \Miat we know of the external world is founded upon experiences 

 gained through our sensory organs, controlled by the judgment. If 

 things exist outside of ourselves which have no inlluence upon our senses, 

 we can form no conception of ibem. Il follows from this pro|Hisition that 

 we can gain knowknlge of the capacity of the sensory organs of animals 

 only by analogy with our own experiences. Elence the distinction of five 

 senses, touch, taste, smell, hearing, and sight, liased upon human pln'siol- 

 logyhas l)een extended to the whole animal kingdom. .1 prior'', however, 

 it cannot be denied that sensations may occur in animals which we do not 



Fig. 70. — Cross-scclion of the human 

 spinal cori.1 (,from W'ie.lersheim). Black 

 represents the gray, white the white sub- 

 stance of the cord; C"<r, central canal, sur- 

 rounded by the anterior and posterior com- 

 missures (C and C) ; Sa, Sp, anterior and 

 posterior fissures; FIE, H\V, anterior and 

 posterior nerve-roots; VH . HH, anterior 

 and posterior horns of gray matter; E, 5, 

 H, anterior, lateral, and posterior columns 

 of white matter. 



