PROTOPLASM 3 



They have been killed by various reagents, selected as causing the most 

 rapid fixation possible. The protoplasm of such cells usually exhibits 

 granules, fibrils, or networks with closed or open meshes. Whether these 

 structures are wholly due to precipitation and coagulation is difficult to 

 determine, but indications that they preexist have been observed in cer- 

 tain living cells. In any case, the various forms of coagulation occur 

 with such constancy that their study is of the utmost importance to the 

 histologist. 



Even the ground substance of protoplasm, in which the fibrils or 

 granules are imbedded, is not necessarily homogeneous. According to 

 Biitschli's interpretation it has the structure oj foam or of an emulsion 

 that is, it consists of minute droplets of one substance completely sur- 

 rounded by walls of another substance. In these walls, granules and fila- 

 ments may be lodged, as seen at the margins of the upper right quadrant 

 of Fig. i. The complex chemical activities of a cell are said to be mani- 

 festly impossible in any homogeneous mass; but in such a heterogeneous 

 medium as an emulsion, they are conceivable (Alsberg). In other words, 

 the vital qualities of protoplasm may not depend so much on hypothet- 

 ical complex and unstable living molecules, as upon the interaction of vari- 

 ous substances, made possible by their arrangement in droplets and invest- 

 ing films. 



The various structures commonly observed in protoplasm may be 

 grouped as follows: 



i. Granules. Ultra-microscopic granules doubtless exist in proto- 

 plasm, since the smallest of those observed approach the limit of visibility. 

 The minute granules, if abundant, give the 



Xissl's bodies. 



protoplasm a dark color. Often they are absent 

 from the peripheral layer of protoplasm, or 

 exoplasm, which is then clear, somewhat firmer, 

 and chemically different from the inner endo- 

 plasm (Fig. i). In addition to minute granules 

 such as may be found in most preserved proto- 

 plasm, certain cells contain larger granules, 

 which are important secretory products elabo- 

 rated by the cell. In active gland cells these 

 granules are well defined and abundant, and F JG . 2 . CLUMPS OF~GRANULES 



.ij..., ., IT i i j (NissL's BODIES) IN A NERVE 



they diminish as the cell becomes exhausted. CELL. 



Various forms of white blood corpuscles may 



be distinguished by the size and staining reaction of the granules im- 

 bedded in their protoplasm. In certain nerve cells (Fig. 2) granules 

 occur in large groups, known as Nissl's bodies. As Crile has shown, 

 these become disorganized as a result of surgical shock or muscular 



