32 HUMAN PHYSIOLOGY. 



of the living bioplasm, but undergoes disruption and oxidation, giving 

 rise at once to heat and force. Coincident with the assimilative 

 processes, a series of disintegrative processes is constantly taking 

 place, whereby the living material is reduced, through a series of 

 downward chemic changes, to simpler compounds, such as water, 

 carbon dioxid, urea, etc. To all these downward changes the term 

 dissimilation, or katabolism, has been given. As a result, also, of 

 these various changes, the protoplasm gives rise to the production 

 of material of an entirely different character, such as globules 

 of fat, granules of glycogen, mucigen, digestive ferments, etc. The 

 sum total of all changes which go on in the cell, both assimilative 

 and dissimilative, are embraced under the general term nutrition, or 

 metabolism. Every cell presents in its nutritive activities an epitome 

 of the nutritive activities of the body as a whole. 



Physiologic Properties of Protoplasm. All living protoplasm pos- 

 sesses properties which serve to distinguish and characterize it viz., 

 irritability, conductivity, and motility. 



Irritability, or the power of reacting in a definite manner to some 

 form of external excitation, whether mechanical, chemic, or electric, 

 is a fundamental property of all living protoplasm. The character and 

 extent of the reaction will vary, and will depend both on the nature 

 of the protoplasm and the character and strength of the stimulus. If 

 the protoplasm be muscle, the response will be a contraction ; if it 

 be gland, the response will be secretion ; if it be nerve, the response 

 will be a sensation or some other form of nerve activity. 



Conductivity, or the power of transmitting molecular disturbances 

 arising at one point to all portions of the irritable material, is also a 

 characteristic feature of all protoplasm. This power, however is 

 best developed in that form of protoplasm found in nerves, which 

 serves to transmit, with extreme rapidity, molecular disturbances 

 arising at the periphery to the brain, as well as in the reverse direc- 

 tion. Muscle protoplasm also possesses the same power in a high 

 degree. 



Motility, or the power of executing apparently spontaneous move- 

 ments, is exhibited by many forms of cell protoplasm. In addition 

 to the molecular movements which take place in certain cells, other 

 forms of movement are exhibited, more or less constantly, by many cells 

 in the animal body e. g., the waving of cilia, the ameboid movements 

 and migrations of white blood corpuscles, the activities of sperma- 



