356 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



however, constituting a marked exception), to the degree of mental disturbance . 

 There are, however, many other sources of increase in the phosphatic compo- 

 nents of the urine ; and hence it is only when these are excluded or allowed for, 

 that the increase attributable to " waste' 7 of Nervous tissue can be estimated. 1 



362. The rapid disintegration of Nervous tissue, when in a state of functional 

 activity, is further indicated by the demand for aliment which this creates; for 

 every one who has been accustomed to habits of sustained mental exertion, must 

 be conscious that, where the general health is good, the appetite for food is no 

 less engendered by such labour, than it is by the exertion of the muscular powers. 

 Further, as already pointed out, there are appearances in the Nervous tissue 

 itself, which indicate that nutritive changes are continually in progress in ite 

 substance ( 346) ; and the rapidity with which it undergoes alteration when its 

 functional operations are suspended, is an additional indication of the activity 

 of the changes of composition (of however different a nature such may be) which 

 those operations involve. 



363. In all that has been said on this subject, reference has been especially 

 made to the " vesicular" element of the Nervous centres; for in regard to the 

 " fibrous" component of the nerve-trunks, which is a mere conductor of the force 

 generated there, the evidence of continual change is not of a kind to justify any 

 such assumption. In fact, there would appear to be strong reasons for believing 

 that the amount of " waste" which it undergoes in the discharge of this office 

 is comparatively trifling ( 355). 



364. Of the actual nature of the changes, by which impressions are received 

 upon the peripheral origins of the afferent nerves, or are communicated to the 

 central origins of the motor, and by which they are conducted along each to 

 their opposite extremities, Physiologists have no certain knowledge. That they 

 are Electrical in their character, has been, and still continues to be, a favourite 

 theory with some ; and the idea seems to derive support from the marked degree 

 in which Electricity, transmitted along the Nervous trunks, can excite the 

 changes to which those nerves are ordinarily subservient. Thus, a feeble gal- 

 vanic current, transmitted along the motor nerves of an animal recently killed, 

 will call the muscles supplied by it into contraction; whilst, on the other hand, 

 a similar current transmitted along an afferent nerve, shall excite reflex move- 

 ments through its ganglionic centre. Further, if the current be transmitted 

 along an afferent nerve, in a living animal, it will excite sensations which are 

 referred to the part whence the nerve arises; and, as will be shown hereafter 

 (CHAP. xv. SECT. 1), Electricity is capable of thus producing sensations of a 

 special kind, as well as those of a general nature. Moreover, in the instanta- 

 neousness of the transmission of Nervous agency from one part of the system 

 to another, there is more analogy to Electricity, than to any other known force. 

 But these and bimilar arguments do not prove the identity of Nervous agency 

 with Electricity; since the effects of the former may be imitated to a certain ex- 

 tent, not merely by Electricity, but by mechanical and chemical stimulation of 

 various kinds. Further, there are powerful arguments against such a suppo- 

 sition, the validity of which cannot be easily set aside. All attempts to prove 



1 Thus, a considerable amount of phosphates, alkaline as well as earthy, passes into the 

 urine directly from the food, without ever becoming part of the living tissue ; so that, if 

 food be withheld, there is an immediate diminution in the quantity which the urinary secre- 

 tion contains a consideration of special importance in studying the influence of acute 

 diseases, in which the quantity of aliment taken is very small. So, again, there are various 

 disordered states of the digestive system, in which there is an excess of phosphates in the 

 urine, without any coincident indication of excessive "waste" of nervous tissue. More- 

 over, the deposit of phosphatic salts is by no means an adequate indication of their presence 

 in excess; since their insolubility may depend upon conditions altogether different. See 

 especially Dr. Golding Bird's treatise on ''Urinary Deposits/' <'IIAJ. x., Am. Ed. 



