24 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



be able to accurately measure them. This we cannot do. We are unable to 

 state in irritation-units the relative value of different kinds of irritants. Even 

 if we could accurately estimate the amount of energy which each form of irri- 

 tant can expend in iritation, we should have only one of the many factors 

 which determine its efficiency. It is equally difficult to compare the irritating 

 effect of irritants upon different forms of protoplasm; e.g. we cannot state 

 what degree of activity of a nerve-fibre corresponds to a certain amount of 

 activity in a muscle-fibre. In spite of the lack of exact quantitative measure- 

 ments, we have gained a clear idea of the way different forms of irritants 

 act when applied to nerves and muscles in certain ways, and have learned to 

 control the methods of excitation sufficiently to permit the influences which 

 alter the irritability of nerves and muscles to show themselves. The effect of 

 irritants can best be studied upon the nerves and muscles of cold-blooded ani- 

 mals, because these retain their vitality ami irritability for a considerable time 

 after they have been separated from the rest of the body. It is a common 

 observation of country folk that the body of a snake remains alive for a long 

 time after the head has been crushed, while the body of a chicken loses all signs 

 of life in a comparatively short time after it has been decapitated. More care- 

 ful examination would show that in neither case do all parts of the body die 

 simultaneously. Each of the myriad cells has a life of its own, which it 

 loses sooner or later according to its nature and to the alterations to which it 

 is subjected by the fatal change. The cells of cold-blooded animals, as the 

 snake and frog, are much more resistant than those of warm-blooded animals, 

 because the vital processes within the cells are less active, and the chemical 

 changes which precede and lead to the death of the part occur more slowly. 

 For instance, the nerves and muscles of a frog remain irritable for many hours, 

 or even days, after the animal has been killed and they have been removed 

 from the body. This fact is of the greatest use to the student. It enables him 

 to study the nerve or muscle by itself, and under such artificial conditions as 

 he cares to employ. Experience shows that the facts learned from the study of 

 the isolated nerve and iniisele hold good, with but slight modification, for the 

 nerves and muscle- when in the normal body. Moreover, it has been found 

 that the nerves and muscles of warm-blooded animals, and even man, resemble 

 physiologically as well as anatomically those of the frog. The correspondence 

 is by no means complete, but it is so great as to make the facts discovered by 

 a study of the nerves and muscles of the frog of the utmost importance to us. 

 We are driven to such sources of information because of the great difficulty of 

 keeping the muscles of warm-blooded animals alive and in a normal condition 

 after removal from the circulation. 



Irritability of Nerves. — The following preparation suffices to illustrate 

 the more striking effects of irritants upon a nerve. A frog is rapidly killed, 

 and then the sciatic nerve is cut high up in the thigh and dissected out. from 

 it- groove, the branches going to the thigh-muscles being divided. The leg is 

 then cut through jusl above the knee. This gives a preparation consisting of 

 the uninjured lower leg and foot, and the carefully prepared nerve supplying 

 tiie muscles of these parts. The leg may be placed foot upward, and fastened 



