114 



GENERAL PRINCIPLES OF ZOOLOGY 



The Tubular System is found only in the chordates (fig. 79). The 

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

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

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

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

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

 a layer of nerve-fibres (the while mailer) ; next is a central portion formed 



of ganglion-cells and nerve-fibres 

 (the gray matter), which is marked 

 off from the central canal by a 

 special epithelium (ependyma). 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 

 the nervous system arises from the 

 ectoderm. Therefore, in many 

 animals, the nerve-cords and the 



FIG. 79. Cross-section of the human 

 spinal cord (from Wiedersheim). Black 

 represents the gray, white the white sub- 

 stance of the cord; Cc, central canal, sur- 



rounded by the anterior and posterior com- 



missures (C and CO; Sa ,Sp anterior and ganglionic masses H e permanently 



posterior fissures; VW, HM , anterior and f 



posterior nerve-roots; VH, HH, anterior in the skin; in others only during 



and posterior horns of gray matter; V, S, the development, later becoming 



H , anterior, lateral, and posterior columns 



of white matter. separated by splitting off or by 



infolding, and thus coming to lie 



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

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

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

 influenced by the will. 



III. Sensory Organs. 



What 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 influence upon our senses, 

 we can form no conception of them. It follows from this proposition that 

 we can gain knowledge of the capacity of the sensor}' organs of animals 

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

 senses, touch, taste, smell, hearing, and sight, based upon human physiol- 

 logy has been extended to the whole animal kingdom. A prior'', however, 

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



