THE PHENOMENON OF CONTRACTION. 55 



although at that temperature much of its contained protein is 

 coagulated. In striated muscle, on the other hand, coagulation 

 may be produced by alcohol without any noticeable shortening. 

 It may be, therefore, that coagulation and shortening are separate 

 results following upon the chemical changes preceding the death 

 of the muscle substance. The coagulation produced in heat rigor 

 is apparently more complete and resistant than that of death rigor, 

 for ordinary death rigor passes off after a certain interval, even if 

 putrefactive processes are excluded; the rigor from heat or from 

 chloroform, on the contrary, shows no release. 



PLAIN OR SMOOTH MUSCULAR TISSUE. 



Occurrence and Innervation. Plain or long striated muscular 

 tissue occurs in the walls of all the so-called hollow viscera of the 

 body, such as the arteries and veins, the alimentary canal, the 

 genital and urinary organs, the bronchi, etc., and in other special 

 localities, such as the intrinsic muscles of the eyeball, the muscles 

 attached to the hair follicles, etc. In structure it differs funda- 

 mentally from cross-striated muscle, in that it occurs in the form 

 of relatively minute cells, each with a single nucleus, which are 

 united to form, in most cases, muscular membranes constituting 

 a part of the walls of the hollow viscera. Each muscle-cell is 

 spindle shaped, contains a single ekmgated nucleus, and the cyto- 

 plasm is traversed by fine fibrils (myofibrillse) which are said to 

 continue from one cell to another. As in the case of the striated 

 muscle, these fibrils are supposed to constitute the contractile 

 element. The muscle-cells are supplied with nerve-fibers belong- 

 ing to the autonomic system (p. 251), which originate directly from 

 so-called sympathetic nerve-cells, and only indirectly, therefore, 

 from the central nervous system. 



Speaking generally, the contractions of this tissue are removed 

 from the direct control of the will, being regulated by reflex and 

 usually unconscious stimulations from the central nervous system. 

 All the important movements of the internal organs, or, as they 

 are sometimes called, the organs of vegetative life, are effected 

 through the activity of this contractile tissue. From this stand- 

 point their function may be regarded as more important than that 

 of the mass of the voluntary musculature, since so far as the mere 

 maintenance of the life of the organism is concerned, the proper 

 action and co-ordination of the movements of the visceral organs 

 is at all times essential. 



Distinctive Properties. The phenomena of contraction shown 

 by plain muscles are, in general, closely similar to those already 

 studied for striated muscle, the one great difference being the 

 much greater sluggishness of the changes. Plain muscles differ 



