STRUCTURE AND ARRANGEMENT OF THE MUSCLES. 545 



plasm, and through the connection of the latter with the interstitial rows of nuclei 

 in the sarcoplasm, or with the protoplasm surrounding the muscle-nuclei, and further 

 with the interstitial cement-substance of the fibrils. 



In the arthropods (crawfish) each muscle-fiber possesses two nerve-termina- 

 tions, arising from separate axis-cylinders. There are no end-plates at the point 

 of entrance; but the nerve-fibrils are distributed for a great distance between the 

 muscle-fibrils, and the nerve-endings appear to extend as far as the terminal or 

 intermediate discs. Some investigators have assumed the existence of a similar 

 arrangement in higher animals also. 



The muscle is supplied also with sensory fibers, which subserve the muscular 

 sense. Their existence is demonstrated physiologically by the fact that stimula- 

 tion of a muscle will cause reflex variations in the blood-pressure and dilatation 

 of the pupil, also an increase in the respiratory movements and muscular reflexes; 

 and, further, by the fact that inflamed muscles are painful. 



At first slender and medullated, these fibers finally become non-medullated. 

 It appears that they are distributed on the outer surface of the sarcolemma, 

 as they wind around the muscle-fibers after undergoing dendritic ramifications. 

 According to others, the sensory nerves, after branching dichotomously, terminate 

 only in the overlying connective tissue or in the aponeuroses, either abruptly or 

 by means of a small swelling. Bremer designates their termination as unbellif erous ; 

 while according to Landauer they pass longitudinally along the muscle-fibers, 

 in the frog, in the form of filaments pro- 

 vided with oval , nuclear formations . Ac- 

 cording to still other observers, they ter- 

 minate as muscular buds or muscular 

 spindles, or as free endings. In the horse 

 the sterno-maxillary muscle receives sen- 

 sory branches from a separate nerve. 



Red and Pale Muscles. In some 

 fish, such as the sturgeon; birds, such 

 as the turkey; and mammals, such as 

 rabbits, two kinds of striated muscles 

 can be distinguished, namely red or 

 dark, for example the soleus and semi- 

 ten dinosus of the rabbit, and pale or 

 light, for example the crural of the rab- 

 bit. The fibers of pale muscle are usu- 

 ally wider, and poorer in protoplasm 

 than those of red muscle; their trans- 

 verse striation is closer, and their longi- 

 tudinal striation less prominent; their 

 fibrils are placed at regular intervals; FIG. 186. Cross-section through the Gastrocnemius 

 their muscle-nuclei, lying directly in ^t&Sf ^L^l^T^'' W ' ^ ^ 



contact with the sarcolemma, are less 

 numerous than those of the fibers of red 



muscle, within which they are situated between the fibrils; and they contain less 

 glycogen, myosin, and water. 



Julius Arnold found in man white fibers extensively distributed among the 

 red. Even in the same muscle, in fact in almost every muscle, in the frog and 

 in mammals, red and white fibers are intermingled (Fig. 186). Nevertheless it 

 should be pointed out that their color is not always clearly differentiated. The 

 physiological differences are considered on p. 563. 



The heart of the frog, as well as that of invertebrates, contains transition- 

 forms between striated and unstriated muscle-fibers (Fig. 184, 6). The spindle- 

 shaped, mononuclear cells have the form of unstriated fibers, but the transverse 

 striation of voluntary muscles. 



Development. Each striated muscle-fiber develops from a mononuclear, 

 mesodermal cell without a wall, which becomes elongated in the form of a spindle. 

 As it progressively increases in length, the nuclei multiply by mitosis. At a more 

 advanced stage the peripheral or parietal substance of this structure is transformed 

 into the fibrillar, striated mass of the fiber (Fig. 184, 7), while the nuclei with a 

 scanty covering of protoplasm (muscle-corpuscles) form a continuous line in the 

 axis of the fiber, where they remain permanently in some animals. In man the 

 nuclei advance later toward the surface of the fiber, where a structureless cuticle, 

 the sarcolemma, separates. The muscle-corpuscles serve in a certain sense as 

 trophic centers for the striated parietal substance ; they may bring about degenera- 

 35 



