MOTION 



45 



nosus) and the latter as pale muscles (adductor magnus). It is readily conceiv- 

 able that this peculiarity in the chemical nature of the dififerent muscles must 



. Fig. 15. — Muscle Fiber of Mammal Highly Magnified, Showing Its Tbansvebse 

 AND Lateral Discs, (a, from Schdfer; b, from Sharpey.) 



lead to differences in the strength and speed of their contraction. Thus, it is 



found that the dark muscles are best 



adapted for the lifting of heavy loads, while 



the pale muscles excel rather by their 



greater rapidity of contraction. The latter, 



however, are more easily fatigued. 



The more primitive smooth muscle tissue''- 

 consists of spindle-shaped cells possessing 

 either a cylindrical or a slightly flattened 

 outline. Their length varies between 45 

 and 225ai and their thickness between 4 and 

 7/i. During pregnancy, the cells of the 

 uterus frequently attain a length of 0.5 mm. 

 Inasmuch as these cells are also composed 

 of a number of fibrillse, they exhibit a deli- 

 cate longitudinal striation. Their nucleus 

 occupies a central position and possesses a 

 long-oval shape which, however, becomes 

 more rounded during the contracted condi- 

 tion of the cell. In its immediate vicinity, 

 as well as in the tapering ends of the cell, is 

 found a considerable amount of undifferen- 

 tiated protoplasm or sarcoplasm. While 

 the striated muscle cells are generally bound 

 together to form compact, rounded masses, 

 the smooth muscle cells are usually em- 

 bedded in a heavy substratum of connec- 

 tive tissue, and the tendency is to spread them out in the form of membranes 



1 McGill, Am. Jour, of Anatomy, ix, 1909. 



Fig. 16. — Sensory Nerve Termi- 

 nations IN Arborizations Around 

 THE Ends of Muscle-Fibers. (Cec- 

 cherelli.) 



