2r)6 THE CAT. [chap. ix. 



(and this, for the most part, is tlieir form in ganglia), many have 

 hut one, very many have two or more — conditions denoted hy the 

 terms "apolar," " luiipolar," "multipolar." Sometimes a process 

 from one cell may he seen to join a process from another cell. 

 Sometimes a process from a cell appears to continue on as the axis 

 cylinder of a nerve. Nerve fihres certainly often appear to end close 

 to cells, hut there is as yet much dispute as to the connexions between 

 them and hetween the processes of different cells. A desire to serve 

 a particular theory has certainly given rise to much exaggeration as 

 to the amount — often even as to the existence — of such connexions. 

 In most cases the processes seem simply to ramify and become finer 

 and finer till they cease to be distinguishable. 



Neuroglia is a substance which immediately surrounds the fibres 

 and cells, and which has been supposed to be a peculiarly modified 

 form of connective tissue. It is a semi-solid matrix which appears 

 granular, though it may really be structureless — its granular 

 appearance being the result of a coagulation. 



Such being the nature and minute constituents of the nervous 

 system, its great mass, the cerebro-spinal axis, is said to be made up 

 of white and of grey nervous matter. The former consists of white 

 fibres only, while the grey matter consists very largely of nervous 

 corpuscles, and is more vascular. Throughout the whole length of 

 the sjnnal part of the cerebro-spinal axis the grey matter lies towards 

 the middle of the whole nervous mass, the white matter being 

 external. In the cerebral part of the same axis, however, the grey 

 matter extends from within and expands over its surface. Although 

 the cerebro-spinal axis is very vascular, yet the arteries and veins 

 which traverse it are very minute. 



6. The cerebro-spinal axis is invested and protected by three 

 MEM15RANES, cnclosed one within the other. 



The first of these is called the dura mater, and is composed of 

 thick, dense, inelastic fibrous membrane, free and smooth on its 

 inner surface, but closely connected externally, in the skull, with the 

 inner surface of the cranial bones, of which it forms the periosteum. 

 In the spinal column it does not constitute the periosteum of the 

 neural canal, but is only connected with the inner surfaces of the 

 vertebra) by loose areolar tissue and fat, and by slips of fibrous 

 membrane. 



In the cranium the dura mater sends inwards two folds of membrane. 

 The first of these is longitudinal, and is called the fair cerebri, 

 and extends from the front of the skull to the occiput, depending 

 from the middle of the cranial vault. 



The second fold is the tentorium, which extends forwards and 

 downwards from the posterior margins of the parietal bones. It is 

 also attached to the upper edge of the petrous portion of each 

 temporal bone. The tentorium, as we have seen in the third 

 chapter, becomes ossified in the adult cat. 



The second of the three membranous envelopes is called the pia 

 mater, and is a very delicate, vascular membrane, which is closely 



