58 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



.-lows chemical processes and increases the endurance. It is noticeable that 

 aerves and muscles remain irritable much longer than ordinarily in ease the 

 body be cooled before their removal. In tie case of a mammal, the irritability 

 may last from six to eighl hours instead of two and a half, while in the ease 

 of frogs it may be preserved at 0° for ten days, although at summer heat it lasts 

 bnly twenty-four hours. In the case of frogs which have been kept at a low 

 temperature the irritability becomes abnormally high when they are warmed 

 to ordinary room-temperature. 



Effect of Chemicals and Drugs. — The irritability of nerve and muscle proto- 

 plasm is markedly influenced by even slight changes in its constitution, If 

 a nerve or muscle be allowed to lie in a liquid of a different composition from 

 its own fluid, and especially if such a liquid be injected into its blood-vessels, 

 an interchange of materials takes place which results in an alteration of the 



i stitution of the tissue and a change in its irritability. Indeed, the only 



solutions which fail to alter the irritability are those which closely resemble 

 serum and lymph. Fluids having other than the normal percentage of salts 

 have a marked effect, while even the absence of proteids appears to have little 

 influence unless continued for a considerable time. 



Pure water acts as a poison to protoplasm, soon destroying its life. 

 Through diffusion and osmosis it is imbibed into the cells at the same time 

 that the salts pass out, and the resulting change in the physical and chemical 

 condition of the tissue cause if rapid, first an increase, and in any case 

 later a decrease, and finally a total loss of irritability. Thus water injected 

 into the blood-vessels of muscles first excites contraction and later destroys 

 the irritability, and results in the condition known as water rigor. These 

 effects are prevented by the presence of small amounts of salt. A sodium 

 chloride solution, of a strength of <> parts per 1000 of distilled water, has 

 been called the physiological solution, because it was supposed to have no 

 effect on the irritability of nerves and muscles of cold-blooded animals; even 

 this solution, if long continued, gradually increases and later decreases the 

 irritability. A solution containing 7 parts of sodium chloride per 1000 is 

 more nearly isotonic to the fluids of cells of the frog, and one containing 9 

 parts per 1000 is approximately in osmotic equilibrium with the fluids of the 

 e<ll- of the mammal. Such fluids cannot be properly regarded as physio- 

 logical solutions, however, for this would mean that they would cause no 

 change in constitution of the cells. They contain only one of the salts essen- 

 tial to the normal activity of the tissues, and the difference in the partial 

 pressure of the other salts of the muscle would cause the muscle cells to lose 

 some of each of these, and, as a result, to have their irritability altered. The 

 importance of the individual salts present in the fluids normally surrounding 

 the tissues, and the need that they should be present in definite proportions, 

 were most strikingly demonstrated by experiments, by Ringer and others, on 

 the nature of the fluid which is essential to the maintenance of the activity 

 of the isolated heart of the frog. These experiments have shown that not 

 only Na, but Ca and K are essential. The heart of the terrapin can be kept 



