DIVISIONS AND STRUCTURE OF THE NERVOUS TISSUE. 505 



CHAPTER XVI. 



PHYSIOLOGICAL DIVISIONS, STRUCTURE AND GENERAL PROPERTIES OF 

 THE NERVOUS SYSTEM. 



Divisions and structure of the nervous tissue Medullated nerve-fibres Simple, or non-medullated nerve- 

 fibres Gelatinous nerve-fibres (fibres of Remak) Accessory anatomical elements of the nerves- 

 Termination of the nerves in the muscular tissue Termination of the nerves in glands Modes of 

 termination of the sensory nerves Corpuscles of Vater, or of Pacini Tactile corpuscles End-bulbs 

 Structure of the nerve-centres Nerve-cells Connection of the cells with the fibres and with each 

 other Accessory anatomical elements of the nerve-centres Composition of the nervous substance 

 Degeneration and regeneration of the nerves Motor and sensory nerves Mode of action of the motor 

 nerves Associated movements Mode of action of the sensory nerves Physiological differences be- 

 tween motor and sensory nerve-fibres Nervous excitability Different means employed for exciting 

 the nerves Rapidity of nervous conduction Personal equation Action of electricity upon the nerves 

 Law of contraction Induced muscular contraction Electrotonus, anelectrotonus and catelectrotonus 

 Negative variation. 



THE nervous system is anatomically and physiologically distinct from all 

 other systems and organs in the body. It receives impressions made upon 

 the terminal branches of its sensory portion and it conveys stimulus to parts, 

 determining and regulating their actions; but its physiological properties 

 are inherent, and it gives to no tissue or organ its special excitability or the 

 power of performing its particular office in the economy. The nervous sys- 

 tem connects into a co-ordinated organism all parts of the body. It is the 

 medium through which all impressions are received. It animates or regu- 

 lates all movements, voluntary and involuntary. It regulates secretion, 

 nutrition, calorification and all the processes of organic life. 



In addition to its action as a medium of conduction and communication, 

 the nervous system, in certain of its parts, is capable of receiving impressions 

 and of generating a stimulating influence, or force, peculiar to itself. As 

 there can be no physiological connection or co-ordination of different parts 

 of the organism without nerves, there can be no unconscious reception of 

 impressions giving rise to involuntary movements, no appreciation of impres- 

 sions, general, as in ordinary sensation, or special, as in sight, smell, taste 

 or hearing, no instinct, volition, thought or even knowledge of existence, 

 without nerve-centres. 



DIVISIONS AND STRUCTURE OF THE NERVOUS TISSUE. 



The nervous tissue presents two great divisions, each with distinct ana- 

 tomical as well as physiological differences. One of these divisions is com- 

 posed of fibres or tubes. This kind of nervous matter is incapable of gener- 

 ating a force or stimulus, and it serves only as a conductor. The other 

 division is composed of cells, and this kind of nervous matter, while it may 

 act as a conductor, is capable of generating the so-called nerve-force. 



The nerve-fibres and cells are also divided into two great systems, as 

 follows : 



1. The cerebro-spinal system, composed of the brain and spinal cord with 

 the nerves directly connected with these centres. This system is specially 

 connected with the functions of relation, or of animal life. The centres pre- 



