THE PHENOMENON OF CONTRACTION. 55 



specific cause of the contraction and coagulation of death rigor 

 nothing final can be said. The interesting researches of Fletcher and 

 Hopkins * indicate that during the survival period between the loss 

 of the normal circulation and the appearance of rigor chemical 

 changes are going on in the living substance which result in the 

 formation and accumulation of lactic acid. When the process of 

 production of the lactic acid ceases, the muscle has lost its irrita- 

 bility, and then soon enters into the stage of rigor. If during this 

 survival period the muscle is kept well supplied with oxygen, no 

 lactic acid accumulates in the muscle, and when the muscle finally 

 loses its irritability, no rigor occurs. These facts would seem to 

 implicate the lactic acid in some way in the process of clotting 

 and of rigor. One specific theory! that is based on the occurrence 

 of lactic acid assumes that the shortening in death rigor, as in 

 normal contraction (p. 74), is due to an imbibition of water by the 

 fibrils in consequence of the action of the acid. Later the acid 

 causes a coagulation or precipitation of the protein, and this process 

 initiates the release from rigor because coagulated protein has less 

 power of imbibition. The fibrils lose water and return to their 

 original length. 



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 elongated nucleus, and the cyto- 

 plasm is traversed by fine fibrils (myofibrillae) 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. 



* Fletcher and Hopkins, "Journal of Physiology," 1907, 35, 247. 

 t Von Furth and Lenk, "Biochemische Zeitschrift," 33, 341, 1911. 



