546 



STRUCTURE AND ARRANGEMENT OF THE MUSCLES. 



tion, or restitution of the latter may arise from. them. The muscles of the young 

 have fewer fibers than those of the adult, although the former are on the whole 

 smaller. In growing muscles, both in the new-born and in later life, the 

 number of fibers is increased by the separation from a fiber of a band of sarco- 

 plasm, together with a continuous row of muscle-corpuscles. This, as a "myo- 

 blast," develops into a new fiber according to the embryonal type. The new fiber 

 also receives its nerve-fiber, which develops from the nuclei of the sheath of 

 Schwann. The individual fibers increase in thickness by an increase in the 

 number of fibrils. In the growth of the muscle as a result of continuous in- 

 creased exertion, the individual fibers become thicker, but 

 not more numerous; the sarcoplasm is increased, while the 

 fibrils and the nuclei are not changed. 



Degeneration. An active degeneration of fibers prob- 

 ably takes place in the muscles, in accordance with their 

 active metabolism. As an introduction to this process 

 (rigid ?) muscular substance accumulates on the fibers in 

 the form of nodules or rings; at such situations the fiber 

 disintegrates into fragments, termed sarcolytes, which un- 

 dergo absorption. 



Comparative. In addition to the parts of vertebrates 

 analogous to human muscuiar tissues, striated muscle- 

 fibers are found also in the iris and the choroid of birds. 



FIG. 187. Unstriated Mus- 

 cle-fibers, isolated by 

 means of Diluted Alco- 

 hol: i, from the intes- 

 tine; 2, from the radial 

 artery of man. 



FIG. 188. Special Forms of Unstri- 

 ated Muscle-fibers from the Mus- 

 cular Coat of the Aorta (after v. 

 Ebner): i, from man; 2, from the 

 hog; 3, from the ox. (The pro- 

 cesses at the sides are cell-bridges 

 that have been torn off.) 



FIG. 189. Muscle-cells from the 

 Frog's Stomach with Distinct Fi- 

 brils (after Engelmann): i, por- 

 tion of a fiber treated with am- 

 monium bichromate; 2, cross- 

 section of cells that have been 

 treated with 8 per cent, sodium- 

 chlorid solution. 



Arthropods have only striated muscle-fibers, while molluscs, worms, and echino- 

 derms have chiefly unstriated fibers. The latter possess also special, energetically 

 contracting fibers with double oblique striation, formed of crossed, oblique lines. 

 In cephalopods the muscle-fibers exhibit spiral lines at the periphery. Among 

 vertebrates fish have the thickest muscle-fibers, then follow with decreasing width, 

 toads, lizards, mammals, and birds. 



The unstriated, involuntary muscle-fibers, or contractile fiber-cells, can be 

 isolated by means of a 35 per cent, solution of potassium hydroxid. They are 

 unencapsulated, unicellular, spindle-shaped, flattened fibers, in some places pre- 

 senting an appearance of longitudinal fibrillar striation, from 45 to 230 // long, 

 and from 4 to 10 // wide. They are occasionally forked at one or both extremities, 

 and contain at the middle a solid, rod-shaped nucleus, which may be sharply 

 brought out by the addition of dilute acetic acid. The nucleus is surrounded by 

 a small amount of protoplasm, and encloses one or two bright nucleoli within a 

 rich network (Fig. 184, 10 and n). The fibers either lie singly, or are joined 

 together in continuous layers or reticular columns, being arranged longitudinally 



