THE CELL AND THE TISSUES. 21 



The foregoing vital manifestations, being chiefly concerned in the 

 mere existence and perpetuation of the cell, are appropriately termed 

 vegetative ; irritability and motion, on the contrary, are the ex- 

 pressions of a higher and more individual existence, and hence are 

 called animal. It is to be remarked that the term "animal," as 

 here employed, must not be regarded as indicating distinctions be- 

 tween plants and animals; for this purpose such manifestations are 

 inadequate, since the elements of certain plants (Mimosese, Dionaea) 

 possess irritability, and the protoplasm of others (Myxomycetes, 

 Volvocineae) exhibits motion in a marked degree. 



Irritability is that property of living matter by virtue of which 

 external influences are responded to by changes within the cell; these 

 changes may, in turn, induce secondary phenomena. Instances of 

 such impressions are frequent among the lower forms, where surface 

 elements, or, as among the still simpler unicellular protozoa, the pe- 

 ripheral zone of the protoplasm common to the entire animal, exhibit 

 susceptibility to external stimuli. Among the higher animals irri- 

 tability is manifested by nerve-cells, which, through their processes, 

 influence other tissues. Concerning the exact nature of the intimate 

 changes taking place within the cell, the sum of which we call nervous 

 phenomena, little is known; it is probable, however, that the al- 

 buminous constituents of the protoplasm are the particular seat of 

 these obscure molecular changes. 



Motion, more or less pronounced, is a characteristic of all ani- 

 mal cells and, likewise, of very many vegetal ones during some 

 portion of their existence. The development and specialization of the 

 adult cell usually result in limitation of the activity of the protoplasm, 

 by reason both of its decrease and of its intimate relations with the 

 surrounding tissues; the cells exhibiting motion in the adult condition 

 are those which retain, to a certain degree at least, their embryonic 

 type: such are the lymphoid and connective-tissue cells. 



Motion may be exhibited by elements devoid of, as well as by those 

 provided with, special appendages. The lowest degree of this vital 

 manifestation is encountered in the streaming of the protoplasm 

 within cells, as in plants, enclosed within limiting membranes which 

 do not permit such motion to affect the exterior of the cells. Con- 

 spicuous examples of the more marked effects of protoplasmic 

 streaming are familiar in the changes readily observed in amoebae Or 

 in the colorless blood-cells of higher types. In these latter elements, 

 however, the motion is manifested rather in change of form than by 

 marked variation in position. 



The highest expression of motion is displayed by those cells whose 

 protoplasm has undergone specialization, resulting either in the pro- 

 duction of a peculiar tissue, as that of the voluntary muscle fibre, or 



