144 



COMPARATIVE ANATOMY 



||S*ia»fe!:-.= 



B 



A 



Fig. 105. — Motor plates. A, from 

 guinea-pig; surface view of muscle fiber: 

 B, from hedgehog; section perpendicular 

 to surface of muscle fiber, g, granular 

 substance of the motor plate; w, striated 

 muscle; n, nerve fiber; t.r., terminal ramifi- 

 cation of the nerve fiber. (From Bremer, 

 Text-book of Histology; after Bohm and 

 Davidoff.) 



which is probably a nutrient medium rather than ordinary cyto- 

 plasm. The wall of the fiber, much more prominent than an ordinary 



cell-wall, is called the sarcolemma. 

 Some investigators maintain that 

 extraneous connective-tissue cells 

 may give rise to a second investing 

 membrane reinforcing the inner 

 sarcolemma which is a product of 

 the fiber itself. 



The alternate dark and light 

 bands on the individual fibril are due 

 to physical differences such that, 

 in polarized light, the dark bands 

 are doubly refractive (anisotropic) 

 while the lighter bands are singly refractive (isotropic). Both the dark 

 and the light bands are traversed by finer markings, as seen under high 



magnification, so that altogether a Nucleus. Sarcoplasm. Fibrils. Lateral branch. 



very complex structure in the fibril 

 is indicated. 



In the act of contracting, pro- 

 found changes occur in the appear- 

 ance and relations of the striations. 

 Undoubtedly the contraction of a 

 fibril is due to specific chemical 

 and physical differentiation within 

 the fibril; the contraction of a fiber 

 is the collective contraction of its 

 fibrils; and the contraction of a 

 muscle is the resultant of the action 

 of its numerous fibers. 



The relation of an unstriated 

 fiber to its nerve is apparently of 

 the simplest sort. A terminal twig 

 of nerve merely attaches to the 

 surface of the fiber, the end of the 

 nerve often showing a knob-like 

 enlargement. Presumably every 

 striated fiber has a nerve connected 

 to it. The nerve, however, enters 

 a small flat plate of nucleated proto- 

 plasm lying superficially on the 

 muscle fiber. Within this motor plate (Fig. 105) the nerve ramifies into 

 fine twigs which seem to terminate in the substance of the plate. 



X Conn, tissue. Capillaries. 

 Fig. 106. — Longitudinal section of a 

 papillary muscle from the human heart. 

 "x" marks the position of intercalated 

 discs. X240. (From Bremer, Text-book 

 of Histology.) 



