THE NERVOUS TISSUE. 107 



The first division, so called as it belongs especially to 

 animals, connects them with the external world, and is 

 hence classed among the organs of relation ; while the 

 second division has its appellation from belonging to the 

 functions of nutrition, which are common to the whole or- 

 ganic world, and constituting organic or vegetative life. 



The first division is situated partly within the cavity of 

 the cranium and vertebral canal, and is the central portion of 

 the system ; while the remaining part extends to the cir- 

 cumference of the body, and is denominated the radiating or 

 peripheral portion. The second division seems to occupy 

 almost exclusively the trunk, and extends in a chain of gan- 

 glia, the whole length of the vertebral column, upon either 

 side. The first division is symmetrical, that is, consists of 

 two equal and similar parts upon either side of the median 

 line of the body ; while the second division is in a great 

 measure destitute of this symmetry. 



In giving a general outline of the nervous tissue, we will 

 adopt that division which is most in accordance with its 

 development, thus connecting its Anatomy with its Physi- 

 ology. The order will be, 



1. The Nerve and Ganglia. 



2. The Spinal Marrow. 



3. Medulla Oblongata. 



4. The Brain. 



The nervous system in animals low in the scale, is seen 

 in the form of a double cord ; a step higher, upon one 

 extremity of this cord are developed knots or ganglia. In 

 the lowest vertebrata, as the fish, five pairs of ganglia are 

 found in succession, upon the anterior extremity of this 

 double cord. In the higher fishes and amphibia, these 

 primitive ganglia have a different disposition. The first 

 two pairs become fused together, forming a single ganglion, 

 or rather are hid by a thin membrane, leaving the three 

 pairs of symmetrical ganglia. This arrangement is traced 

 upwards into the mammalia, where, as in the dog, is seen 

 this single ganglion constituting the cerebellum, and the 



