54 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



lation of lactic acid, that it is, so to speak, a last tonic contraction, 

 the acid bringins; into action the mechanism of shortening as it does 

 in a normal contraction (p. 73). In addition to the rigor mortis 

 that occurs after death at ordinary temperatures, a condition of 

 rigor may be induced rapidly by raising the temperature of the 

 muscle to a certain point. Rigor induced in this way is designated 

 as heat rigor or rigor caloris. Much uncertainty has prevailed as to 

 whether heat rigor is different essentially from death rigor. Ac- 

 cording to some physiologists, the processes may be regarded as the 

 same, the heat rigor being simply a death rigor that is rapidly devel- 

 oped by the high temperature, this latter condition accelerating 

 the chemical changes leading to rigor, as is the case, for instance, 

 in the action of chloroform. This ^^ew is supported by a study 

 of the chemical changes that take place under the two condi- 

 tions, as will be described later, and by the fact that some of 

 the conditions that influence one phenomenon have a parallel 

 effect upon the other. For instance, death rigor is acceler- 

 ated by previous use of the muscle, and the same is true for heat 

 rigor. 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 proc- 

 esses 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 elongated 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. 248), 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 



