92 ZOOLOGY. 



OF THE NERVOUS SYSTEM. 



181. This system is formed by a soft pulpy substance, 

 which is almost fluid in early life, but acquires more consis- 

 tence with years. The aspect of this tissue, called nervous, 

 varies much ; occasionally white, at other times grey or ash- 

 coloured ; in some parts it forms masses or ganglions, in 

 others elongated cords. The cords are called nerves; the 

 masses, ganglions or nervous centres. 



182. In man, and in those animals which approach him 

 in structure, the nervous apparatus is composed of two parts 

 the cerebro- spinal or nervous system of animal life, the gan- 

 glionic or nervous system of organic life ; and each of these 

 systems is composed in its turn of two parts a central, com- 

 posed of the nervous masses, and a peripheric, composed of 

 the nerves which proceed from these centres to all parts of 

 the body (Fig. 55). 



183. Cerebro- spinal System in Man. The central 

 portion of this system, also called the cerebro-spinal axis or 

 encephalon, is composed essentially of the cerebrum, cere- 

 bellum, and medulla spinalis, and is lodged in an osseous 

 cavity and canal, formed of the cranium and spinal column. 



184. Envelopes of the Encephalon. Three membranes 

 enclose, support, and nourish the cerebro-spinal axis. The 

 first is the dura mater, a fibrous membrane of considerable 

 strength, adhering to several points of the osseous canal, and 

 forming a strong protecting covering for the encephalon. It 

 sends prolongations towards the interior of the cranium, 

 serving to protect various parts of the organ ; and in its sub- 

 stance are formed various venous canals or sinuses, in com- 

 munication with the general venous system of the body. 

 These sinuses are called sinuses of the dura mater. 

 Within the dura mater is a second membrane, called the 

 arachnoid. This is a serous membrane of extreme tenuity, 

 and transparent, but firm, and like all serous membranes, is 

 a sac without any opening into it. By one layer it invests 

 the inner surface of the dura mater, and by the other the 

 brain, thus providing for the movements of the organ. 



Beneath the cerebral layer of the arachnoid is the pia 

 mater, immediately investing the nervous tissue itself. This 

 is a cellulo-vascular membrane, in which ramify the vessels 

 proceeding to and from the brain ; for the arteries especially, 

 are, in general, minutely subdivided before they actually 





