PHYSIOLOGICAL ANATOMY OF THE NERVOUS TISSUE. 565 



In treating of the nervous system, we shall consider first the physiological anatomy 

 of the nervous tissue ; next, the general properties of the cerebro-spinal system ; next, 

 the functions of different portions of this system connected with motion, ordinary sensi- 

 bility, intellection, etc. ; next, the functions of the sympathetic, or organic system of 

 nerves ; and finally, the special senses, with the physiological anatomy and mechanism 

 of the accessory parts. 



Physiological Anatomy of the Nervous Tissue. 



The physiological anatomy of the nervous system naturally divides itself into two 

 sections ; one embracing what is called the general anatomy of the nervous tissue, and 

 the other, the arrangement of this tissue in special organs, as far as this is connected 

 with their functions. 



The intimate structure of the different portions of the nervous system may now be 

 regarded as tolerably well understood, at least so far as those anatomical points bearing 

 upon physiology are concerned. The connection between the nerve-cells and the fibres 

 and the modes of termination of the motor filaments in the muscles are points nearly if 

 not quite settled ; and the terminations of sensory filaments in integument and mucous 

 membranes have lately been investigated very thoroughly and with quite positive and 

 satisfactory results. These anatomical points are specially connected with the general 

 properties of the nervous system, both as a generator of the so-called nerve-force and as 

 a conductor. 



The arrangement of the nervous elements in special organs, as in the brain and spinal 

 cord, has not been so successfully investigated and presents immense difficulties in its 

 study ; and we can hardly hope to acquire any thing like a definite and thorough knowl- 

 edge of the functions of these parts, until we have much more positive information con- 

 cerning their anatomical characters. 



Anatomical Divisions of the Nervous Tissue. The physiological division of the 

 nervous system into nerves and nerve-centres is pretty well carried out as regards the 

 anatomical structure of these parts. The two great divisions of the system, anatomically 

 considered, are into nerve-cells and nerve-fibres. 



The nerve-cells, as far as we know, are the only parts capable, under any circum- 

 stances, of generating the nerve-force ; and, as a rule, they cannot receive impressions 

 in any other way than through the nerve-fibres. There are, however, some exceptions, 

 either apparent or real, to this rule, as in the case of direct irritation of the ganglion of 

 the tuber annulare, portions of the cerebrum, and the sympathetic ganglia, which seem 

 sensible to direct irritation ; but the cells of most of the ganglia belonging to the great 

 cerebro-spinal axis are insensible to direct stimulation and can only receive impressions 

 conducted to them by the nerves. 



The nerve-fibres act only as conductors and are incapable of generating nerve-force. 

 There is no exception to this rule, but there are differences in the properties of certain 

 fibres. The nerves generally, for example, receive direct impressions, the motor fila- 

 ments conducting these to the muscles and the sensory filaments conveying the impres- 

 sions to the centres. These fibres also conduct the force generated by the nerve-centres. 

 But there are many fibres, such as those composing the white matter of the encephalon 

 and the spinal cord, that are insensible to direct irritation, while they convey to the 

 centres impressions made by the sensitive nerves and conduct to the motor nerves the 

 stimulus generated by nerve-cells. 



Structure of the Nerves. There are few anatomical elements that present greater 

 variations in size and appearance than the nerve-fibres. Certain fibres found in the 

 course of the nerves between the muscles are as large as T?TZ of an inch, have dark 

 borders, and possess three well-marked structures, viz., a tubular membrane, medullary 

 contents, and an axial band ; others, with the same structure, are only ^^- of an inch 



