TEE NERVOUS SYSTEM IN GENERAL. 653 



The nerve-cells have prolongations or poles, whose number varies from one 

 to hve. Cells with only one prolongation are named unipolar; those which 

 have two are bipolar ; and those which have a greater number are designa- 

 ted multipolar. These poles establish relations with the nerve -tubes and 

 constitute the origin of the nerves. 



Fig. 311. 



MULTIPOLAR OR STELLATE GANGLIONIC NERVE-CELL, WITH ONE OP ITS 

 PROLONGATIONS — 



a, Becoming continuous with the axis-cylinder of a double-contoured nerve-iibre, b. 



Such are the anatomical elements that enter into the structure of the 

 nervous system. 



In the white substance of the cerebro-spinal axis, only meduUated nerve- 

 tubes of every size are found ; in the grey substance are tubes, and a more 

 or less considerable number of nerve-cells are situated along their course. 



To these two elements is added a large quantity of hlood-vessels, which 

 are incomparably more abundant in the grey than in the white substance. 



In the nervous cords, the elementary tubes are alone met with ; they are 

 disposed in long bundles, which are collected into successively increasing 

 fasciculi. A cellulo-vascular envelope, the neurilemma (or perineurium), 

 binds all these fasciculi into a single cord, and forms a special sheath 

 around each of them. The details of their organisation will be referred to 

 hereafter. 



It is admitted that the ganglionic, or nerves of organic life, possess a 

 greater quantity of slender tubes than the others. These tubes are com- 

 monly designated as the organic nerve fibres. They also contain fibres of 

 Eemak. 



In the ganglia, the cells are joined to the nerve-tubes. It has been shown, 

 by dissection and microscopical observation, that the corpuscles composing 

 the ganglia at the origin of the trunks are all attached to the superior 

 fibres. The other tubes have none. 



In the cerebrospinal axis, the two substances are equally associated with 

 each other, but in a variable manner, according to the region. In the cord 



