2/2 PHYSIOLOGY. 



general sensations already referred to, such as hunger, 

 thirst, fatigue, nausea, satiety, faintness, etc. They are 

 often called "common sensations," and Martin designates 

 them as " sensations which we do not mentally attribute to 

 the properties of external objects, but to the conditions of 

 our own bodies." 



General Sensations. Nerve endings in different parts 

 of the body may be affected by the blood and the lymph, 

 and give us sensations of comfort, discomfort, restlessness, 

 fatigue, faintness, etc. These are called general sensa- 

 tions. They are probably due to the condition of the 

 blood, or to the condition of nutrition of the various parts 

 of the body. Thus after muscular exercise the muscles are 

 acid in their reaction, while they are alkaline after resting ; 

 after exercise carbon dioxid accumulates in them to a cer- 

 tain extent. Hunger and thirst come on after abstinence 

 from food and drink, or after work exhausting the tissues. 

 The presence of the various waste products, or the condi- 

 tion of the cells as the result of their activity, acting 

 through the nerve endings in the tissues, keep the nerve 

 centers informed as to the condition of the parts of the body. 

 If these conditions are extreme, we may have definable sen- 

 sations, but ordinarily the sensations are of an undefinable 

 sort which we designate as " general sensations." 



The Muscular Sense. As an example, we will take 

 the case of estimating the weight of an object by holding 

 it in the hand. Our estimate is thought by some to be the 

 result of (i) direct consciousness of the degree of effort 

 put forth ; but probably it is (2) a sensation, or complex of 

 sensations, aroused by nerve impulses from the organs 

 used. There are afferent nerve fibers with endings in 

 (i) the skin, (2) the muscles and tendons, (3) the joints, 



