STKUCTURE OF THE NERVOUS SYSTEM. 437 



which are not prolonged into nerve-fibres. It is not easy to specify 

 the function of the former; bat since the processes of the latter 

 apparently fulfil the functions of nerves, it is probable that they 

 bring different regions of the central organs into mutual connection. 

 Cells of this kind exist in the spinal cord and the brain every- 

 where, in large amount, but not in the ganglia. 



The cells in the gray matter of the brain, which is regarded as 

 the seat of the mental manifestations, exhibit no peculiarities by 

 our present means of research. 



While, therefore, we may regard cells connected with. nerve-fibres 

 as being either motor or afferent, the functions of the apolar, and 

 the multipolar not thus connected, must be established by future 

 investigations. 



Development of the Nerve- Cells. 



The nerve-cells are merely transformed primordial cells ; some 

 simply enlarging, while others throw out a varying number of pro- 

 cesses, and some of them are connected with nerve-fibres. 



Many nerve-cells also appear at a subsequent period to increase 

 by division, from the fact that two nuclei sometimes occur in the 

 nerve-cells of young animals. 



Valentin thinks he observed regeneration of nerve-cells in the 

 superior cervical ganglia of the rabbit. Gluge also maintains that 

 the gray matter of the brain is reproduced after being removed. 



Pathological States and New Formations of the Nerve-Cells. 



The deposit of pigment-granules becomes excessive in the cells 

 of the brain in old persons. Fatty deposition also occurs in them 

 a fatty degeneration. 



The ganglia become atrophied in old persons ; in which case the 

 ganglion cells are less numerous, and contain more pigment. Atro- 

 phy of the abdominal ganglia also occurs as a sequel of typhus. 

 (Raciborski^ 



A new formation of gray nerve-cells sometimes occurs on the 

 walls of the cerebral ventricles. ( Virchow.) 



SECTION II. 

 STRUCTURE OF THE NERVOUS SYSTEM. 



The nervous system consists of 



I. The nerves proper, and their ganglia. 

 II. The nervous centres. 



