246 Physiology. 



these nerves to the brain. But ordinarily we are not con- 

 scious of them. If the body is in need of food, the mes- 

 sages are stronger and we have a sensation of hunger. If 

 the poisonous waste matters are not removed by the organs 

 of excretion, their presence in the lymph is reported to the 

 brain, and we have a feeling, perhaps of fatigue, or of 

 decided discomfort, or even of pain. 



The Muscular Sense. In judging the weight of a body 

 by holding it in the hand, our estimate is the result of sen- 

 sations aroused by nerve impulses from the organs used. 

 There are afferent nerve fibers with endings in (i) the skin, 

 (2) the muscles and tendons, and (3) the joints. In hold- 

 ing out the arm and in moving it up and down, all three 

 of these sets of nerve endings are stimulated, and impulses 

 are conveyed to the brain producing the muscular sense. 



Dependence of Sight on Muscular Sense and Touch. It is difficult to 

 realize the importance of the muscular sense. An illustration of the as- 

 sistance which touch and the muscular sense give to the sense of sight 

 is furnished in the case of a boy who had been blind from birth, and re- 

 ceived sight at the age of twelve years by means of a surgical operation. 

 At first he could not distinguish a globe from a circular card of the same 

 color until he had touched them. He knew the peculiar features of the 

 dog and the cat by feeling, but not by sight. Happening one day to 

 pick up the cat, he recognized for the first time the connection between 

 the new sense of sight and the old familiar ones of touch and the mus- 

 cular sense. On putting the cat down he said, " So, puss, I shall know 

 you next time." 



Pain. The nerves of general sensibility give informa- 

 tion of the state of nutrition in the tissues and the condi- 

 tion of the body as a whole. Ordinarily we are not aware 

 of these nerve currents. When they become stronger 

 than usual they give rise to feelings of general discomfort, 

 such as fatigue, depression, restlessness, etc. When the 

 currents become stronger still, we have pain. 



