THE NERVE SYSTEM. 



THE SPINAL COED AND BRAIN, WITH THEIR 



MENINGES. 



THE nerve system of man is an apparatus by means of which he appreciates 

 and becomes influenced by impressions from the outer world, reacts on 

 these impressions, and hence is enabled to adapt himself to his environment. 

 It is the organic substratum for those manifestations of nerve force engaged in the 

 characteristic attributes of animal life .sensation and motion. Broadly stated, 

 the nerve system connects the various parts of the body with one another and 

 coordinates them into one harmonious whole in order to carry on the bodily 

 functions methodically and to control the physiological division of labor through- 

 out the organism. With the evolution of the higher forms of animal life through 

 an immense phylogenetic past the nerve system has undergone remarkable differ- 

 entiation and specialization, attaining its maximum as to dominant position and 

 complexity of structure in the human species. 



The description of the nerve system is assisted by the accommodation of physio- 

 logical data to the anatomical basis in order to demonstrate more clearly and 

 to render more practical our knowledge of the mutual relations of its structure 

 and function. The cycle of events which accompanies nerve action is determined 

 by impressions received by the peripheral organs, apperception and reflexes 

 of these impressions in the lower nerve centres, correlation of these with other 

 impressions in higher centres, as well as voluntary reactions or inhibitions, liber- 

 ated in compliance with the organic or higher needs of the individual. 



Conventionally, the nerve system is usually considered as consisting of (1) 

 the cerebrospinal system, comprising (a) the central nerve axis (brain and spinal 

 cord) and (/;) the peripheral nerves (cranial and spinal), and (2) the sympathetic 

 nerve system. This subdivision, like others formulated by various authors, 

 is an arbitrary one. Xo part of the system stands isolated, and the manifold 

 groupings and chainings of the units of the system intimately connect the central 

 nerve organs with the peripheral nerve endings, the organs of special sense and 

 the vegetative organs. The distinction between the central and the sympathetic 

 systems has been too absolute, and the only justification for adhering to the 

 classification given above is based upon the fact that the sympathetic system is 

 preponderatingly related to the interconnection and coordination of the nutritive 

 (digestive, respiratory, and blood and lymph) apparatus, and, therefore, exer- 

 cises a special control over its activities. 



Structurally considered, the nerve system consists of cell-elements peculiarly 

 differentiated from all other tissue cells in that their protoplasm is extended, often 

 to great distances from the nuclear region, in the form of processes. The cell- 

 elements are held in place by supporting tissues, partly of ectodermal and partly 

 of mesodermal origin, and receive an abundant blood supply. 



The cell element of the nerve system is called the neurone. The neurone is 



(807) 



