336 THE SOLIDS. 



ART. XVIII. MUSCLE. 



FEW of the animal tissues have been more extensively 

 examined than the muscular: the multiplied observations 

 made on its structure have, however, led neither to that uni- 

 formity of opinion respecting it, nor, indeed, to that accurate 

 knowledge of its minute anatomy which might have been 

 anticipated; of the truth of this position, evidence will be 

 shortly adduced. 



Muscles admit of division into the voluntary, or those 

 which are under the control of the will, and the involuntary, 

 or those of which the action takes place independently of 

 the will ; the former consist of the muscles of animal life, 

 those, for example, of locomotion, and the latter embrace 

 those of organic life, as the muscles of the alimentary canal 

 (the sphincters of the oesophagus and anus excepted, which 

 are to a certain extent voluntary), the heart, the uterus, the 

 bladder, &c. 



It will be observed, that the involuntary muscles, or 

 those of organic life, usually encircle the hollow viscera: 

 there are some other situations, however, in which involun- 

 tary muscular fibres are met with, as in the trachea and its 

 bronchial ramifications, the iris, the sarcolemma, and, ac- 

 cording to some observers, as Bowman, they are also en- 

 countered in the dartos and covering the excretory ducts 

 of the larger glands, as the ductus communis choledochus, 

 the ureters, and vasa deferentia ; and it is with them a 

 matter of question how far the contractility of the skin, 

 and the erection of the penis, clitoris and nipple, may be de- 

 pendent upon the presence of involuntary muscular fibrilla?. 

 In the preceding article I have, however, shewn that the 

 contractility of these parts depends upon the presence of a 

 nucleated form of elastic tissue allied to unstriped muscular 

 fibre in many of its properties, but yet distinct therefrom. 



