CHAPTER IV 



MUSCULAR TISSUE 



GENERAL CONSIDERATIONS 



Muscular tissue consists essentially of protoplasm in which the gen- 

 eral vital property of contractility has become predominant. However, 

 the path of contraction is practically limited to one direction, the long 

 axis of the cell. This phenomenon of contractility results from the dif- 

 ferentiation of specially contractile fibrils, the myofibrils, from the pro- 

 toplasm of embryonic muscle elements, the myoblasts. The protoplasm 

 of the muscular tissue is called sarcoplasm. Adult muscular tissue may 

 be divided into three classes: smooth, cardiac, and striped. All three 

 types arise from mesoderm, with the exception of the dilator and sphinc- 

 ter muscles of the iris of the eye, and the muscle of the secretory portion 

 of the sweat gland both of the smooth variety which are generally be- 

 lieved to be of ectodermal origin. In lower forms, muscle tissue may be 

 largely derived from the ectoderm and even from the entoderm. 



The smooth muscle is in general limited to the viscera ; it is not under 

 the control of the will, hence also called involuntary muscle. The cardiac 

 type is limited to the heart, and to the middle layer of the roots of the 

 aorta, pulmonary artery, and pulmonary veins. It is striped, but like 

 smooth muscle, controlled by the sympathetic nervous system; therefore 

 independent of the will, hence also of involuntary type. So-called striped 

 or skeletal muscle is practically limited to the skeleton, and subserves the 

 function of skeletal movement. This group includes also the muscles 

 of the eyeball, the ear, the upper third of the esophagus, diaphragm, 

 and tongue. It is under the control of the will, hence designated volun- 

 tary. The striped muscle of the diaphragm and the esophagus is appar- 

 ently only partially voluntary. 



It is obvious from the above that there is demanded a more specific 

 terminology: involuntary smooth (unstriped) ; involuntary striped (car- 

 diac) ; and voluntary striped (skeletal). The three types pass through 

 very similar, perhaps identical, earlier stages of histogenesis. The essen- 



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