558 



SIGNIFICANCE OF THE NERVOUS SYSTEM 



The Structural Unit of the Nervous System. — In conformity with 

 other tissues, nervous tissue is composed of two types of cells which 

 may be characterized as true and accessory. The former are called 

 neurons and constitute the functional element of this tissue, while the 

 latter are used for the supporting framework composed of ependyma 

 and neuroglia or glia cells. It is true, however, that these cells are 

 indispensable to one another, i.e., one cannot in all probability exist 



without the other, but looked 

 at in a general way, it is the 

 neuron which attracts our atten- 

 tion most, because it is more 

 directly concerned with nervous 

 processes. In the terminology 

 of Waldeyer,^ the neuron or 

 nerve-cell is the histological unit 

 of the nervous system, and as 

 such includes the cell-body as well 

 as its protoplasmic processes, 

 namely, the dendrites, axis 

 cylinder, arborizations and col- 

 laterals. Looked at in this way, 

 the nervous system consists of 

 enormous numbers of neurons^ 

 supported by glia cells and en- 

 veloped here* and there by pro- 

 tective membranes, such as the 

 dura mater, arachnoid and pia 

 mater. This constitutes the 

 ''neuron concept" of the nervous 

 system. We shall see later on 

 that the structural independence 

 thus granted to the neuron, is 

 associated with an unmistakable 

 physiological distinctiveness. 



The External Characteristics 



of the Neuron. — Neurons are 



cells modified to suit a particular 



purpose, namely, that of generat- 



FiQ.274. — Normal Anterior Horn Cell Show- ing and conducting nerve im- 



(Warrington.) pulses. They are in reality 



neuroplasmic fibers possessing 



an accumulation of cytoplasm 



at one point of their course in which are embedded a nucleus and 



nucleolus. In this regard they present the essential details of a cell, 



^ Deutsche med. Wochenschr., xvii, 1891, 1244. 



2 Kolliker objects to this term upon philological grounds without, however, 

 furnishing a more correct or convenient concept. 



mo THE NissL Granules. 

 a, The Axon. 



