12 



General Part. 



enveloped by a thin sheath, the sarcolemma, which is wanting in 

 smooth as well as in striped muscle cells. The fibres are usually- 

 cylindrical, and are rounded, rarely branched or forked, at the ends ; 

 they are often of considerable length. In the Arthropoda the whole, 



and in the Vertebrata the greater 

 part, of the musculature consists of 

 striated muscle fibres, which like the 

 striped muscle cells, contract with 

 greater rapidity, and with greater 

 force, than the smooth muscle cells. 

 Muscle cells and fibres may not only 

 form large tracts of tissue, but they may 

 also occur as isolated secondary constitu- 

 ents of the connective tissue; where 

 these are very numerous and the connective 

 tissue is scanty, an appearance of muscular 

 tissue is produced. This shows the inti- 

 mate relation between connective tissue 

 cells and muscle cells, which is further 

 demonstrated, in the case of the scattered 

 muscle elements, by the occurrence of 

 connective tissue cells which have been 

 partially modified into muscle cells. Some- 

 times, like epithelial cells, muscle cells are 

 held together by cement substance. 



4. Nervous tissue. The 

 contraction of muscle cells* is 

 brought about by stimuli received 

 from ganglion cells, each of 

 which has a thread-like prolongation often of considerable length 

 (Fig. 15, 2). At its free end each of these processes breaks into a 

 tuft of branches which lie closely upon the muscle cell ; sometimes 

 a process gives off branches on its course, and these are attached to 

 muscle cells. Besides these long processes, the ganglion cell may 

 also give origin to numerous shorter branching offshoots, which do 

 not pass to muscle cells. 



Ganglion cells of this description are called motor: there 

 is another kind, the sensory (Pig. 15, 4), which are, externally, 

 just like the motor ones, but receive, by their long processes, 

 impressions from the outer world. The process may, for instance, 

 go to the epithelium covering the surface of the body, and branch 

 between its cells. {See the section on Sense Organs, p. 18). 



The ganglion cells occur in groups, comprising both kinds. They 

 are attached to one another by some of the prolongations ; those 

 of one cell do not as a rule, however, pass directly to another, but 



Pig. 14. Connective tissue cell (1), 

 smooth muscle cells (3 — 5), and a, cell 

 (2) which is undergoing transformation 

 into a muscle cell. Prom the urinary 

 bladder of the Salamander. — After 

 riemming'. 



* Under muscle cells, smooth and striped muscle cells and also muscle fibres are 

 included. 



