112 PHYSIOLOGY. 



the forehead, from whence the flap was taken, until the new vas- 

 cular and nervous communications are established. 



An active capillary circulation in a part is essential to its sen- 

 sibility ; any cause which retards this, deadens the sensibility of the 

 part, as is well seen in the benumbing influence of cold. Increased 

 vascular action, again, produces a corresponding increase in sensi- 

 bility ; this is seen in the active congestion preceding inflammation. 



Sensations are divided into general and special. General sensa- 

 tion is distributed over all the body ; by it we feel those impressions 

 made upon our bodies by surrounding objects, which produce the 

 various modifications of ^jam and pleasure, the sense of contact and 

 resistance, and variations of temperature. Special sensation is that 

 which arises from impressions of a peculiar character, upon nerves 

 which are adapted to receive them alone. 



The intensity of all sensations is very much blunted by frequent 

 repetition, excepting in the case of those to which the attention is 

 particularly directed ; these, so far from losing their acuteness, be- 

 come much more cognizable by the mind. Hence arises the educa- 

 bility of the special senses. 



Although there are some stimuli which can produce sensory im- 

 pressions on all the nerves of sensation, it will be found that those, 

 to which any one organ is peculiarly fitted to respond, produce little 

 or no efi^ect upon the rest. Thus the ear cannot distinguish luminous 

 rays, nor the eye the undulations of sound ; and the same is true of 

 the other senses. Hence it may be inferred, that no nerve of special 

 sensation can, by any possibility, take on the function of another. 

 But that each requires its own peculiar stimulus to call it into 

 actio?!, light for the eye, and sound for the ear, &c. 



The nerves of special sensation have in themselves no general 

 sensibility ; they may be pricked or torn without the individual suf- 

 fering any pain ; they only experience or give rise to their own 

 peculiar sensations. All the general sensibility that the organs of 

 the senses possess is derived from nerves of general sensibility dis- 

 tributed to them. The special senses are five in number, viz. : touch, 

 taste, smell, hearing, seeing. To these some add a sixth, the mus- 

 cular sense, or thai by which the will can produce, check or regulate 

 the amount of contraction in the voluntary muscles ; and also appre- 

 ciate, by certain sensations originating in the muscles, the precise 

 degree of contraction in each. 



The organs of the special senses consist of two parts, a physical 

 and a vital part. The physical part receives and modifies the 

 impression ; the vital transmits the impression to the brain. The 

 different transparent media of the eye contribute its physical portion ; 

 the nerve is the vital portion. 



