MUSCULAR TISSUE. 147 



muscle, and later the other parts of the sarcomeres (Godlewsky). 

 It may be stated that, according to J. B. MacCallum (and other 

 observers), the heart-muscle develops from spindle-shaped cells 

 lying close together in the protoplasm of which there is found a 

 fairly regular network. As development proceeds, fibrils or fibril 

 bundles which run parallel to the long axis of the cells make their 

 appearance at the nodal points of this network. 



The muscle-cells of the so-called fibers of Purkinje lie immediately 

 beneath the endocardium, and are remarkable in that their proto- 

 plasm is only partially formed of transversely striated substance, and 

 that only at their periphery. Such cells are found in great numbers 

 in some animals (sheep), but rarely in man. Heart muscle has a 

 rich blood supply, which will be considered more fully when the 

 heart is discussed as an organ. 



For the nerve-endings in smooth and striated muscle-fibers see 

 the chapter on Nervous Tissues. 



TECHNIC. 



Fresh, striated muscle-fibers may be isolated by teasing them in 

 an indifferent fluid. After a short time the sarcolemma may separate as 

 a very fine membrane. If a freshly teased muscle be placed in a cold 

 saturated solution of ammonium carbonate, the sarcolemma will become 

 detached in places within five minutes (Solger, 89, III). 



Striated muscle-fibers may be examined in an extended condi- 

 tion by placing an extremity in such a position as to stretch certain 

 groups of muscles. A subcutaneous injection of 0.25-0.5 c.c. of a i$> 

 osmic acid solution is then made. The acid penetrates between the fibers 

 and fixes them. Pieces of muscle are then cut out and washed in dis- 

 tilled water. Teased fibers, even if not stained, will show the stria- 

 tion plainly if mounted in glycerin. Muscles thrown into a state of 

 tetanic contraction by electric stimulation may also be fixed in this state 

 and later examined. 



Cross-sections of muscles, extended and fixed in osmic acid, 

 also show the relation of the fibrils to the sarcoplasm (Cohnheim's fields). 

 A remarkable quantity of sarcoplasm in proportion to the number of 

 fibrils is seen, for instance, in the muscles which move the dorsal fin of 

 hippocampus ; among the mammalia a similar condition is found in the 

 pectoral muscles of the bat (Rollett, 89). 



In the muscles of all adult vertebrates (except the mammalia) 

 the nuclei lie between the fibrils. In young mammalia they also have 

 this position, but in the adult animals only the nuclei of red muscles 

 are found between the fibrillse ; in all other muscles the nuclei are under 

 the sarcolemma. 



The fibrillar structure of muscle-fibers can be seen by teasing old 

 alcoholic preparations, or tissue treated with weak chromic acid (0.1%) 

 or one of its salts. 



In alcoholic preparations of mammalian muscle, the cross- 

 striation is clearly seen, and is intensified by staining with hematoxylin. 

 This stain colors everything anisotropic in the muscle, but does not affect 

 the remaining structures. Similar results may be obtained with other 



