90 FUNCTIONS OF THE NERVOUS SYSTEM. 



the term is employed in regard to other operations of the hodily frame, tn 

 general, by the function of an organ, we understand some change which may 

 be made evident to the senses, as well in our own system as in the body of 

 another. Sensation, Thought, Emotion and Volition, however, are changes im- 

 perceptible to our senses, by any means of observation we at present possess. 

 We are cognizant of them in ourselves, without the intervention of those pro- 

 cesses by which we observe material changes external to our minds ; but we 

 judge of them in others, only by inferences founded on the actions to which 

 they give rise, when compared with our own. When we speak of sensation, 

 thought, emotion, or volition, therefore, as functions of the Nervous System, 

 we mean only that this system furnishes the conditions under which they 

 take place in the living body; and we leave the question entirely open, 

 whether the tyxn nas or nas not an existence independent of that of the mate- 

 rial organism, by which it operates in Man as he is at present constituted. 



109. In regard to the second class of actions, it may be remarked, that they 

 are nearly all connected, more or less closely, with the organic functions, or 

 with the protection of the body from danger. Thus the movements of the 

 pharynx supply to the stomach the alimentary materials it prepares for the 

 nutrition of the body; those of the muscles of the thorax, &c., maintain that 

 constant interchange of air in the lungs, which is necessary for the aeration 

 of the blood ; whilst those by which a limb is involuntarily retracted from any 

 cause of pain or irritation, are obviously adapted to the latter of these two ends. 



II. Elementary Structure of the Nervous System. 



110. Wherever a distinct Nervous System can be observed, it is found to 

 consist of two kinds of structure ; the presence of both of which, therefore, 

 may be regarded as essential to our idea of it as a whole. One of these is 

 that which is designated the white or fibrous matter. This constitutes (with 

 the neurilema or nerve-sheath, and the areolar tissue which it encloses), the 

 whole of the nervous trunks, wherever they occur ; and forms a large part of 

 the central masses with which they are connected. It consists of tubes of 

 great minuteness, which are composed of an interlacement of extremely deli- 

 cate fibres ; some of these passing in a longitudinal, and some in a transverse 

 or spiral direction. When these tubes are examined immediately after death, 

 their contents appear pellucid and homogeneous, and of a fluid consistence ; 

 so that each tube or fibre 'looks like a cylinder of clear glass, with simple, 

 well-defined, dark edges. But a kind of coagulation soon takes place in the 

 contained substance, which makes it easily distinguishable from the tube 

 itself; for the latter is then marked by a double line, as shown in Fig. 7. 

 This last state of the nervous fibre has been regarded by Remak and others 

 (but probably erroneously) as the natural one ; and the substance contained 

 within the tube has been described as a band or axis, composed of several 

 distinct filaments. It has even been asserted that, at the extremities of 

 these nerves, the filaments diverge from one another, and form loops, the 

 tubular envelop being lost ; but this is probably erroneous. The walls of the 

 tube are not unfrequently seen to contain the nuclei of the cells, by the coa- 

 lescence of which it was originally formed. The diameter of the cylindrical 

 tubuli of the nerve-trunks is estimated to vary from about the l-2000th to the 

 1 -3000th of an inch. The fibres gradually decrease in size, however, as they 

 approach the brain, either directly, or through the medium of the spinal cord ; 

 and in the brain itself they continue to grow less as they pass through the 

 medullary to the cortical part ; so that in the former they measure from 

 1 -7000th to l-8000th of an inch, and in the latter not more than l-14000th of 

 an inch. The fibres of the olfactory and optic, and, in a less degree, those of 



