THE NERVOUS SYSTEM IN GENERAL. 739 



vertebral canal, forms the spinal cord. It is a large white cord, terminating in 

 a point at its posterior extremity, and giving off, at each intervertebral foramen, 

 one of those nerve-branches which, collectively, represent the peripheral portion 

 of the apparatus of innervation. 



The expanded extremity enclosed in the cranium is named the encephalon (or 

 brain). More complicated in its conformation than the spinal cord, this portion 

 is divided, as we shall see, into four parts : 1. A white peduncle, the continuation 

 of the spinal cord. 2. Three grey-coloured ovoid masses, one of which is posterior, 

 the other two being anterior, and placed symmetrically side by side. This 

 medullary prolongation gives off, right and left, like the cord itself, nerve- 

 branches destined almost exclusively to the head. 



The Nerves. — The nerves are in the form of fasciculated cords, issuing 

 from the orifices at the base of the cranium, or through the intervertebral 

 foramina, and passing into all the organs by ramifying like arteries, which they 

 generally accompany. 



All the nerves arise from the medullary axis, or from its encephalic pro- 

 longation, by radicles more or less apparent. They are divided, according to 

 the relative position of their point of emergence, into two great categories — the 

 superior, arising from the corresponding face of the spinal axis ; the others, 

 inferior, escaping from the lower face — a distinction which is perfectly appre- 

 ciable with regard to the cord itself, but which is more difficult to estabhsh in the 

 encephalic peduncle, as it is less distinct. 



At their emergence from the bony canals which give them passage, the 

 radicles of each nerve always unite into a thick common trunk. 



In the majority of cases, there enter into the composition of this trunk the 

 nerves or fibres of the two orders ; only a few nerves are composed of fibres of 

 the one kind, and these all belong to the brain. 



At the origin of the trunk into which the nerve roots are collected, there is 

 a greyish enlargement termed a ganglion ; but this peculiarity belongs exclusively 

 to the superior fibres. 



After a variable course, which is generally short, this trunk divides into 

 branches, the point of departure for all the nerves of the body. Among these 

 branches, those which are expended in the apparatuses of animal life are pairs, 

 and perfectly alike on both sides of the body. Those of the organs of nutrition 

 are composed at first of an almost symmetrical double chain, placed beneath the 

 spinal column, the elements of which are borrowed from nearly all the nerve 

 trunks issuing from the cerebro-spinal axis ; in proceeding to their destination, 

 their distribution is most irregularly complicated. As they offer on their course 

 & great number of ganglia similar to those we have already mentioned, they are 

 called ganglionie nerves ; they are also designated the net'ves of organic or vegetative 

 life, while the others are named the nerves of animal life or of relation. 



Structure of the Nervous System. 



Two particular substances — one greg, the other white — enter into the organi- 

 sation of the nervous apparatus. These two substances are formed, the first of 

 nerve-tubes and united nerve-cells ; the second of tubes alone. 



The nerve-tubes are microscopic elements, composed of sections or segments 

 about a millimetre in length, joined end to end. Each segment {inter anyiular 

 segment) is formed of a sheath {Schivann's sheath) — a thin, homogeneous mem- 



