148 THE TISSUES. 



stains, such as basic anilin dyes, but not with the same precision as with 

 hematoxylin. 



A certain species of beetle {Hydrophilus^) is admirably adapted 

 for the study of the finer details of striation. The beetle is first wiped dry 

 and then immersed alive in 93% alcohol. On examining in dilute 

 glycerin after from twenty -four to forty -eight hours, the substance of 

 its muscles will show disintegration into Bowman's discs (vid. p. 141). 

 The latter swell up in acids and are finally dissolved, as may be 

 seen, by adding a drop of formic acid to a specimen prepared as above 

 (Rollet, 85). 



In order to study the relation of muscle to tendon, small mus- 

 cles with their tendons are put into a 35% potassium hydrate solution for 

 a quarter of an hour, after which the specimen is placed upon a slide and 

 teased at the line of junction of the two tissues. This will separate the 

 muscle-fibers from their respective tendon-fibrils (Weismann). 



Similar results may be obtained by immersing a frog in water 

 at a temperature of 55 C., in which the animal soon dies with muscles 

 perfectly rigid. As soon as the water begins to cool (one-quarter hour) 

 the frog is removed and a small piece of its muscle cut out and teased in 

 water on a slide (Ranvier). 



Cardiac muscle-cells are isolated by maceration for twenty-four 

 hours in a 20% solution of fuming nitric acid (potassium hydrate with a 

 specific gravity of 1.3 will do the same in one-half or one hour). The 

 margins of the cells may be brought more clearly into view by placing 

 pieces of heart muscle for twenty-four hours in a 0.5% aqueous solution 

 of silver nitrate and then cutting into sections. 



Isolated fibers of Purkinje are obtained by immersing pieces 

 of endocardium (0.5 mm. in size) in 33% alcohol and then teasing 

 them on a slide. The sheep's heart is especially well adapted for this 

 purpose. 



Nonstriated muscle-fibers are isolated in the same way as heart 

 muscle. In thin cross-sections (under 5 ^ in thickness) of intestinal 

 muscle, preferably of a cat, fixed in osmic acid, the intercellular bridges 

 may be seen here and there between the fibers. 



D. THE NERVOUS TISSUES. 



The entire nervous system, peripheral as well as central, is com- 

 posed of cells possessing one or many processes. These cells 

 develop early in embryonic life from certain ectodermal cells (nenro- 

 blasts] of the neural canal, which is formed by a dorsal invagination 

 of the ectoderm. The neuroblasts soon develop processes, many 

 of them in loco, others only after wandering from the neural canal. 



The processes of the nerve-cells are of two kinds : (i) un- 

 branched processes having a nearly uniform diameter throughout, 

 with lateral offshoots known as collateral branches ; these, as we 

 shall see, generally form the central part of a nerve-fiber, and are 

 known as neuraxes (Deiters' processes, axis-cylinder processes, 

 neurites, neuraxones or axones) ; and (2) processes which branch 

 soon after leaving the cell-body and break up into many smaller 



