6 HUMAN ANATOMY. 



the single cell constitutes the entire organism, and as such is capable of performing 

 the functions essential for the life-cycle of the animal. 



As the embryonal cell advances in its life-history, the conditions to which it is 

 subjected induce, with few exceptions, further specializations, since in all but the 

 lowest forms division of labor is associated with a corresponding differentiation and 

 adaptation to specific function. 



Vital manifestations of the cell include those complex physico-chemical 

 phenomena which are exhibited during the life of the cellular constituents of the 

 body in the performance of the functions necessary for fulfilment of their appointed 

 life-work. These embrace metabolism, growth, reproduction, and irritability. 



Metabolism, the most distinctive characteristic of living matter, is that process 

 by which protoplasm selects from the heterogeneous materials of food those partic- 

 ular substances suitable for its nutrition and converts them into part of its own sub- 

 stance. Metabolism is of two forms, — constructive and destructive. Constructive 

 metabolism^ or anabolism^ is the process by which the cell converts the simpler com- 

 pounds into organic substances of great chemical complexity; destructive vietabolism, 

 or katabolism, on the contrary, is the process by which protoplasm breaks up the 

 complex substances resulting from constructive metabolism into simpler compounds. 

 Vegetal cells possess the power of constructive metabolism in a conspicuous degree, 

 and from the simpler substances, such as water, carbon dioxide, and inorganic salts, 

 prepare food-material for the nutritive and katabolic ])rocesses which especially dis- 

 tinguish the animal cell, since the latter is dependent, directly or indirectly, upon 

 the vegetal cell for the materials for its nutrition. 



Growth, the natural sequel of the nutritive changes effected by metabolism, 

 may be unrestricted and equal in all directions, resulting in the uniform expansion 

 of the cell, as illustrated in the growth of the ovum in attaining its mature, condition. 

 Such unrestricted increase, however, is exceptional, since cells are usually more or 

 less intimately related to other structural elements by which their increase is modi- 

 fied so as to be limited to certain directions ; such limitation and influence result in 

 unequal groivth, a force of great potency in bringing about the differentiation and 

 specialization of cells, and, secondarily, of entire parts and organs of the body. 

 Familiar examples of the result of unequal growth are observed in the columnar 

 elements of epithelium, the fibres of muscular tissue, and the neurones of the ner- 

 vous system. 



Reproduction may be regarded as the culminating vital manifestation in the 

 vegetative life-cycle of the cell, since by this process the parent element surrenders 

 its individuality and continues its life in the existence of its offspring. While the 

 details of the process by which new cells arise from pre-existing cells are reserved for 

 consideration in connection with the more extended discussion of the cell to follow 

 (see page 9), it may here be stated that reproduction occurs by two methods, — 

 the indirect or mitotic and the direct or amitotic. The first of these, involving the 

 complicated cycle of nuclear changes collectively known as mitosis or karyokinesis, 

 is the usual method ; the second and simpler process of direct division, or amitosis, 

 is now recognized as exceptional and frequently associated with conditions of im- 

 paired vital vigor. 



Irritability is that property of living matter by virtue of which the cell ex- 

 hibits changes in its form and intimate constitution in response to external impres- 

 sions. These latter may originate in mechanical, thermal, electrical, or chemical 

 stimuli to which the protoplasm of even the lowest organisms responds, or they may 

 be produced in consequence of the obscure and subtle changes occurring within the 

 protoplasm of neighboring elements, as illustrated by the reaction of the neurones in 

 response to the stimuli transmitted from other nervous elements. 



THE ANIMAL CELL. 



Ever since the establishment of the Cell Doctrine, in 183S, by the announcement 

 of the results of the epoch-making investigations of Schleiden and Schwann on "The 

 Accordance of Structure and Growth of Animals and Plants," the critical examination 

 of the cell has been a subject of continuous study. Notwithstanding the tireless enthu- 



