838 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



stimulation of its peripheral ends ; or, if the nerve-trunk be divided, the 

 most powerful irritants may be applied to the peripheral area in which 

 that nerve is distributed without calling forth sensation. 



When an impression is made upon a sentient surface, that impres- 

 sion so changes the molecular equilibrium in the terminal filaments as to 

 give rise to afferent impressions, which travel along the nerve-trunk to 

 reach the centres of the brain, and it is the final change which occurs 

 within the nerve-cells which is to be spoken of as sensation, and it is 

 only this latter change of which the brain is cognizant. 



Two kinds of sensibility may be recognized, — general and special 

 sensibility. Nerves of general and of special sense are concerned in the 

 perception of these two kinds of sensation. By the term special sensi- 

 bility is understood that sensation which arises from an impression of a 

 peculiar kind and special character, which is capable of affecting only 

 one kind of nerve; or, rather, which, when applied to one kind of nerve, 

 will invariably produce a sensation peculiar to that nerve. Thus, for 

 example, a stimulus applied to the terminal filaments of the nerve of 

 hearing will invariably be recognized as an auditor} - sensation ; so a stim- 

 ulus of the optic nerve, whether mechanical, electrical, or chemical, will 

 be recognized as a visual sensation. 



By general sensibility is meant the appreciation of sensations arising 

 from impressions of a general character applied to the general tegumen- 

 tary surface of the bodj-. Under this head are to be included the tactile 

 sense, the sense of heat and cold, and the muscular sense. The special 

 sensations are the sensations of smell, taste, sight, and audition. 



Certain conditions are necessary to sensibility. First, there must 

 be a certain degree of vascularity. Unless the part be well supplied with 

 blood it cannot be endowed with either general or special sensibility. 

 An illustration of this maj r be seen on tying any large artery ; the part to 

 which it is supplied becomes almost instantly numb ; the blood is cut off', 

 the vascularity diminishes, and the irritability of the receptive filaments 

 of the nerve becomes depressed. So, also, cold, as is well known, 

 reduces sensibility by diminishing the blood-supply of the part. 



Secondly, mere vascularity is not, however, sufficient ; there must, 

 likewise, be continuity with the nerve-trunk. Unless the afferent fibres 

 be in such continuity with the centre no sensibility, either general or 

 special, may take place. While, finally, the centre itself must be in a 

 state of integrity to recognize or convert into perceptions the 

 impressions brought to it through afferent nerves. 



When impressions have been frequently repeated upon nerves of 

 general or special sensibility the sensations to which they ordinarily ad- 

 minister become blunted. An illustration of this may readily be drawn 

 from the nerves of special sense. Sights or sounds constantly present 



