20 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



As was said before, animal protoplasm is pre-eminently katabolic, and the 

 evidence of its katabolism is found in the waste products, sucb as C0 2 , 

 II.O, and area, which arc given off from animal organisms. Assimilation 

 and disassimilation, or anabolism and katabolism, go hand in hand, and 

 together constitute an ever-recurring cycle of activity that persists as long 

 as the material retains its living structure, and is designated under the 

 name metabolism. In most forms of living matter metabolism is in some 

 way self-limited, so that gradually it becomes less perfect, old age comes on, 

 and finally death ensues. It has been asserted that originally the metabolic 

 activity of protoplasm was self-perpetuating — that, barring accident, the cycle 

 of changes would go on forever. Resting upon this assumption it has been 

 suggested by Weissmann that the protoplasm of the reproductive elements 

 still retains this primitive and perfect metabolism and thus provides for the 

 continuity of life. The speculations bearing upon this point will be discussed 

 in more detail in the section on Reproduction. 



Reproduction in some form is also practically a universal property of 

 living matter. The unit of structure among living organisms is the cell. 

 Under proper conditions of nourishment the cell may undergo separation into 

 two daughter cells. In some cases the separation takes place by a simple act 

 of fission, in other cases the division is indirect and involves a number of 

 interesting changes in the structure of the nucleus and the protoplasm of the 

 body of the cell. In the latter case the process is spoken of as karyokinesis 

 or mitosis. This act of division was supposed formerly to be under the con- 

 trol of the nucleus of the cell, hut modern histology has shown that in kary- 

 okinetic division the process, in many cases at least, is initiated by a special 

 structure to which the name centrosome has been given. The many-celled 

 animals arise by successive divisions of a primitive cell, the ovum, and in the 

 higher forms of life the ovum requires to be fertilized by union with a sper- 

 matozoon before cell-division becomes possible. The sperm-cell acts as a 

 stimulus to the egg-cell (see section on Reproduction), and rapid cell-division 

 is the result of their union. It must be noted also that the term reproduc- 

 tion includes the power of hereditary transmission. The daughter-cells are 



similar in form to the parent-cell, and tl rganism produced from a fertilized 



ovum is substantially a facsimile of the parent forms. Living matter, there- 

 fore, not only exhibits the power of separating off other units of living matter, 

 but of transmitting to its progeny its own peculiar internal structure and 

 properties. 



Contractility and conductivity are properties exhibited in one form or 

 another in all animal organisms, and we must concede that they are to be 

 counted among the primitive properties of protoplasm. The power of con- 

 tracting or shortening is, in fact, one of the commonly recognized features of 

 a living thing. It is generally present in the simplest forms of animal as 

 well as vegetable life, although in the more specialized forms it is found most 

 highly developed in animal organisms. The opinion seems to be general 

 among physiologists that wherever this property is exhibited, whether in the 



