234 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



fibrinous clot in their interior, diminution of sensibility, consequent upon 

 the insufficient circulation, is one of the first symptoms. 



404. On the other hand, increased circulation of blood through a 

 part produces exaltation of its sensibility; that is, the ordinary im- 

 pressions produce changes of unusual energy in its sensory nerves. 

 This is particularly evident in the increased sensibility of the genital 

 organs of animals during the period of heat ; and in those of Man, 

 when in a state of venereal excitement. Moderate warmth, friction, 

 exercise, and other causes which increase the circulation through a part, 

 also augment its sensibility ; and this augmentation is one of the most 

 constant indications of that state of determination of blood, or active 

 congestion, which usually precedes inflammation, and which exists in the 

 parts surrounding the centre of inflammatory action. But it must be 

 borne in mind, here as elsewhere ( 401), that such exaltation of func- 

 tion in a limited part, is quite consistent with general debility ; and in 

 fact we may often observe, that the tendency of such local affections 

 is particularly great, when the blood is in a very poor condition. (See 

 CHAP, v.) 



405. To sum up, then, we may compare the vesicular substance, 

 wherever it exists, to a galvanic combination : the former being capable 

 of generating nervous influence, and transmitting it along the fibrous 

 structure, to the part on which it is to operate ; in the same manner 

 as the latter generates electric power, and transmits it along the con- 

 ducting wires, to the point at which it is to effect a decomposition or 

 any other change. In one of the most perfect forms of the galvanic 

 battery (that invented by Mr. Smee) although the metals remain in- 

 serted in the acid solution, and are consequently always ready for 

 action, no electricity is generated until the circuit is complete ; and the 

 waste of the zinc, produced by its solution in the acid, is therefore 

 exactly proportional to the electric effects to which it gives rise. The 

 condition of the nervous system, in the healthy and waking state, bears 

 a close analogy to this ; for it is in a state constantly ready for action, 

 but waits to be excited ; and its waste is proportional to the activity of 

 its function. The vesicular matter, diffused over the surface of the 

 body, is inactive, until an impression is made upon it by some external 

 agent ; but a change then takes place in its condition (of which we 

 know no more, than that the presence of arterial blood and a certain 

 amount of warmth are necessary for it), which is transmitted to the 

 central organs by the sensory trunks. It would appear that the excite- 

 ment of this change has a tendency to increase the afflux of blood to 

 the part ; thus when a lozenge or some similar substance is allowed to 

 lie for a time in contact with the tongue, or with the side of the mouth, 

 a roughness is produced, which is due to the erection of the sensory 

 papillae, by the distension of their blood-vessels. On the other hand, 

 the change in the vesicular matter of the central organs, by which 

 motion is produced in the distant muscles, may be excited either by the 

 stimulus conveyed by the afferent nerves (as in reflex action, 392), or 

 by an act of Mind. This act may be voluntary, originating in the will ; 

 or it may be instinctive or emotional, resulting from certain states of 



