CHAPTEK XL 



MUSCULAR ACTION. 



THE sense-organs which we have just briefly studied are at the begin- 

 ning of the typical reflex arc in the nervous system. They originate the 

 impulses which pass into the cord and the brain. The organs which we 

 are now to study for a little are at the other end of the reflex arc and are 

 the chief means by which the individual accomplishes his purposes. 

 These organs are the muscles found in nearly all parts of the body. 

 Homologous to them as active instruments are the glands, and these we 

 have already considered. In general terms the function of the muscles 

 is to cause the approximation of two parts of the body or to constrict a 



FIG. 223 



FIG. 224 



<*> c 



Epithelio-muscular cells from a hydra: q, 

 muscular prolongations from the cell-body. 

 (Dubois.) 



hollow tube or viscus. Biological 

 histology deals with the minute 

 structure of various contractile or- 

 gans, and to the text-books of this 

 science the reader is referred. In 

 some of the simplest animals, even 

 in the Protozoa (for example, Sten- 

 tor), one already finds contractile 

 organs. Here they are in the form 

 of very delicate threads technically 



called myoids, and it is their sole business to cause the animal to quickly 

 shorten. It is likely that something very similar to the myoids of the 

 simplest animals persists in all muscular tissue, in the elaborately evolved 

 cross-striated variety as well as in smooth muscle. The intimate structure 

 of the three sorts of muscle found in man (cross-striated, cardiac, and 

 smooth muscle) must be thoroughly understood in connection with this 

 chapter. Knowledge also of the methods of grouping of the contrac- 

 tile cells into muscle-bundles, of those within the anatomical muscle, and 



A diagram suggesting how muscle-fibers 

 originate: a, a formative cell or myoblast; 

 b and c, stages in development of contractile 

 fibers out of the undifferentiated protoplasm in 

 a frog. (R. Hertwig.) 



