GENERAL CONSIDERATIONS. 563 



CHAPTER XVII. 



PHYSIOLOGICAL DIVISIONS, STRUCTURE, AND GENERAL PROPERTIES OF THE 



NERVOUS SYSTEM. 



General considerations Divisions of the nervous system Physiological anatomy of the nervous tissue Anatomical 

 divisions of the nervous tissue Medullated nerve-fibres Simple, or non-medullated nerve-fibres Gelatinous 

 nerve-fibres (fibres of Eemak) Accessory anatomical elements of the nerves Branching and course of the nerves 

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

 sensory nervesCorpuscles of Pacini, or of Vater Tactile corpuscles Terminal 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 Eegeneration of the nervous tissue Eeunion of 

 nerve-fibres Motor and sensory nerves Distinct seat of the motor and sensory properties of the spinal nerves- 

 Experiments of Magendie upon the roots of the spinal nerves Properties of the posterior roots of the spinal nerves 

 Properties of the anterior roots of the spinal nerves Recurrent sensibility Mode of action of the motor nerves 

 Associated movements Mode of action of the sensory nerves Sensation in amputated members General prop- 

 erties of the nerves Nervous irritability Different means employed for exciting the nerves Disappearance of 

 the irritability of the motor and sensory nerves after exsection Nerve-force Rapidity of nervous conduction 

 Estimation of the duration of acts involving the nerve-centres Action of electricity upon the nerves Induced 

 muscular contraction Galvanic current from the exterior to the cut surface of a nerve Effects of a constant gal- 

 vanic current upon the nervous irritability Electrotonus, anelectrotonus, and catelectrotonus Neutral point- 

 Negative variation. 



I 



THE nervous system is anatomically distinct in all animals except those lowest in the 

 scale of being. It is useless to speculate upon the question of the existence of matter 

 endowed with properties analogous to those observed in the nervous system of the higher 

 animals, in beings so low in their organization as to present no divisions into anatomical 

 elements ; for the present condition of physiological science does not admit of the recog- 

 nition of functions without organs. All animals that present any thing like nervous 

 functions present also an anatomically distinct nervous system. Within certain limits, 

 the perfection of the animal organization depends upon the general development of the 

 nervous system. 



High in the animal scale, as in the warm-blooded animals, the general development of 

 the nervous system presents little if any variation ; but special attributes are coexistent 

 with the development of special organs. The development in this way of particular por- 

 tions of the nervous system is in accordance with the peculiar conditions of existence of 

 different animals ; it is a necessary part of their organization, and is not dependent upon 

 education or intelligence. Examples of this are in the extraordinary development of the 

 sense of sight, hearing, or smell, in different animals. There are animals in which these 

 special senses possess a delicacy of perception to which man, even with the greatest 

 amount of intelligent education, can never attain ; but man, possessing a nervous organi- 

 zation not superior to that of other warm-blooded animals in its general development, 

 and inferior to many in the development of special organs, stands immeasurably above 

 all other beings, by virtue of the immense preponderance of what is known as the 

 encephalic portion of the nervous system. 



These brief general considerations will convey some idea of the physiological impor- 

 tance of the nervous system ; of tHe care which should be exercised in its study ; and of 

 the great interest attached to it, from the fact that the most complex and important of 

 its functions belong to human physiology, and to human physiology alone. 



We can best define what is to be included under the head of the nervous system, by 

 citing certain of its prominent and well-established properties and functions : 



1. The nervous system is anatomically and physiologically distinct from all other sys- 

 tems 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 the opera- 

 tion of their functions; but its physiological properties are inherent, and it gives to no 



