THE SENSES 855 



there, dividing into branches and losing their medullary sheath, form 

 endings of various kinds around and between the muscular fibres. It 

 is probable that in contraction of the muscles the nerve-fibres in the 

 spindles are compressed, and thus mechanically stimulated. 



In the spinal cord these impulses are conducted up through the 

 posterior column ; and, although nothing is known as to the paths 

 they follow in the higher parts of the central nervous system, it is 

 certain that there is some afferent bond of connection between the 

 cortical motor areas and the muscles which they control (p. 754). 



Tactile sensations set up in the skin or mucous membrane lying 

 over contracting muscles may also help the nervous motor mechanism 

 in appreciating and regulating the amount of contraction ; but the 

 fact that, in anaesthesia of the mucous membrane covering the vocal 

 cords produced by cocaine, the voice is not at all impaired, shows 

 that muscular contractions of extreme nicety can be carried on 

 without any such aid. 



Fain. 



Pain has been defined as * the prayer of a nerve 'for pure blood.' 

 The idea is not only true as poetry, but, with certain deductions and 

 limitations, true as physiology. That is to say, pain, as a rule, is a 

 sign that something has gone wrong with the bodily machinery ; 

 freedom from pain is the normal state of the healthy body. Physio- 

 logically, pain acts as a danger-signal ; it points out the seat of the 

 mischief, and even, in certain cases, by compelling rest, favours the 

 process of repair. Thus, the surgeon has sometimes looked upon 

 pain as ' Nature's splint.' But as a matter of fact, a certain amount 

 of pain occurring at intervals is not incompatible with high health j 

 and probably nobody, even when accidents and indiscretions of all 

 kinds are avoided, is entirely free from pain for any considerable 

 time. Sometimes, indeed, the mere fixing of the attention on a 

 particular part of the body is sufficient to bring out or to detect a 

 slight sensation of pain in it ; and it is matter of common experience 

 that a dull continuous pain, like that of some forms of toothache, is 

 aggravated by thinking of it, and relieved when the attention is 

 diverted. 



In general, the skin is far more sensitive to pain than the deeper 

 structures. The most painful part of an operation is generally the 

 stitching of the wound. The cutting of healthy muscle causes no 

 pain. In an operation in which an artificial connection was estab- 

 lished between the stomach and the small intestine (gastro-enteros- 

 tomy), and in which no anaesthetic was administered, the only pain 

 of which the patient complained was produced by the incision in the 

 skin (Senn). The spasmodic contraction of the intestines and 

 stomach causes the intense pain of colic and gastralgia. Labour is 

 an example of a strictly physiological function which is the occasion 

 of severe pain. Tissues normally insensible to pain may become 

 acutely painful when inflamed. 



