84 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



pressure of oxygen is diminished. All protoplasmic motion is rapidly 

 arrested in a vacuum. 



4. Various Chemical and Physical Agents. — Various chemical agents 

 are capable of modifying the contractility of protoplasm. Thus, a slight 

 excess of acid or of alkali will arrest protoplasmic movement; hence, 

 protoplasmic motions in the cells of various vegetable organisms, such 

 as cara, will be arrested, after two or three minutes, in a one-tenth of one 

 per cent, soda solution. Dilute acids cause coagulation of protoplasm, 

 and will perhaps explain the poisonous action of carbon dioxide and the 

 necessity of its removal from cells as rapidly as formed. Various poisons, 

 such as ether and chloroform, interfere with the activity of protoplasm 

 of all forms, and the similarity of action serves to still further demon- 

 strate the identity of protoplasm. Thus, the alkaloid veratrine produces 

 effects on all forms of protoplasm similar to those so well marked in 

 muscular tissue. 



Protoplasm, also, like muscular and other irritable tissue, responds 

 to various forms of artificial stimuli, though the degree of susceptibility 

 to such stimuli may vary in different forms of protoplasm. Electrical 

 currents, when powerful, are capable of killing protoplasm, causing it to 

 assume a spherical shape, and to become opaque, shriveled, and granular. 

 Feeble currents slow the spontaneous motions of protoplasm, while 

 strong currents arrest them. Where the contractile tissue .is inclosed 

 in tubular sheaths and the assumption of the. spherical form so rendered 

 impossible, as in muscular tissue, an attempt is made to approach the 

 form as nearly as possible. Such protoplasmic cylinders when stimulated 

 become shorter and thicker. 



Sudden changes of temperature are also capable of producing either 

 increase or decrease in the contractility of protoplasm, the change being 

 more marked the more rapidly the variation in temperature occurs. 



Absence of light also serves finally to arrest protoplasmic motion, 

 while its presence will lead to increased vigor of contraction, as already 

 referred to in the changes in the contractile pigmented cells of the skin 

 of the chameleon and frog. 



