62 MUSCULAR TISSUE. 



ries, veins, lymphatics, sweat-glands connected with hair-follicles, 

 scrotum, and areola of the nipple of the breast. 



The nerves of involuntary muscle end in plexuses or networks, 

 as in the cardiac muscle. 



Development of Involuntary Muscular Tissue. The contractile 

 fiber-cells which compose this tissue are formed from cells of the 

 mesoblast, which elongate, the nuclei also elongating. The 

 muscular tissue of the sweat-glands is formed from the epiblast. 

 When new muscular tissue of the plain variety is formed, as when 

 the uterus enlarges in pregnancy, growing from an organ weighing 

 from 30 to 40 grams to one weighing from 900 to 1100 grams, 

 this is accomplished by an increase in the size of the original 

 fibers, and by the formation of new fibers from small cells which 

 lie between the original ones. In the process of involution, that 

 process by which the uterus returns to its original size, the fibers 

 become fatty and are absorbed. 



Chemical Composition of Striated Muscular Tissue. The sarco- 

 lemma resembles elastin. When the contractile substance is 

 pressed, a fluid is expressed, the muscle-plasma, which coagulates, 

 the clot being myosin. A similar change takes place after death, 

 producing rigor mortis or cadaveric rigidity. During life muscular 

 tissue has an alkaline reaction ; while after death, owing par- 

 tially, at least, to the formation of sarcolactic acid, it becomes acid. 

 This also occurs after the muscles have been very active. 



PERCENTAGE COMPOSITION OF HUMAN MUSCLES. 



Water 73.5 



Proteids, including the sarcolemma, proteids of connective 



tissue, vessels, and pigments 18.02 



Gelatin 1.99 



Fat . . 2.27 



Extractives 0.22 



Inorganic salts 3.12 



The proteids in muscle-plasma are three in number : 1. Para- 

 myosinogen, which coagulates at 47-50 C., constituting 17 to 

 22 per cent, of the total proteid ; 2. Myosinogen or Myogen, coag- 

 ulating at 56 C., 77 to 83 per cent. ; and traces of an albumin, 

 Myo-albumin. Both paramyosinogen and myosinogen enter into 

 the clot which forms when the plasma coagulates. This clot is 

 called myogen-fibrin or myosin-fibrin. 



The extractives are very numerous, creatin, creatinin, xan- 

 thin, hypoxanthin, carnin, carnic acid, uric acid, tannin, and 

 inosinic acid, all containing nitrogen and fats, glycogen, inosit, 

 dextrose, and sarcolactic acid. This acid is attributed by some 

 authorities to the glycogen, while others trace it to the proteids. 

 The presence of urea in mammalian muscular tissue is still a 

 matter of dispute. Muscular tissue always contains fat, and there 

 is excellent authority for believing that, while some of this comes 



