494 Dynamic Theory. 



alimentary canal, including the throat, stomach and intestines, and the 

 urinary duct, consists in the contraction of the transverse fibres, or those 

 which run around the tube of the intestines, or other canal ; such con- 

 traction beginning in the upper end, and being followed successively by 

 the contraction of the fibres below ; the effect being to drive the con- 

 tents of the organ before the slowly moving wave of contraction. At 

 the same time the longitudinal muscle fibres, by their contraction, tend 

 to pull up the tube over its contents, which are carried down with it as 

 the contraction relaxes. Smooth muscle constitutes the walls of blood 

 vessels, some of the fibres running parallel with the tube, and others 

 around, as in the case of the alimentary canal; their contractions which, 

 in the case of arteries, appear to be controlled by some sort of automatic 

 machinery ( perhaps internal pressure of contents ), tend to diminish the 

 size of the tube and retard the flow of blood into the capillaries. 



When smooth muscle is irritated by electricit} 7 , the stimulus spreads 

 across from one fibre to another, showing no insulation of fibres, but 

 when the striated fibre is stimulated in the same way, the stimulus with 

 accompanying contraction confines itself to the fibres receiving the 

 charge, unless the irritation is very strong, showing the sarcolemma to 

 be to a certain extent an insulator. 



The heart is an involuntary organ, but is peculiar in being composed 

 of striated muscles which, however, are destitute of sarcolemma, so that 

 an irritation applied to any part of the organ is spread over and excites 

 the rest. Another peculiarity is in the fact that the heart muscle fibres 

 are branched, in which oddity it imitates the muscles of the tongue of 

 the frog, which are also branched like a tree. 1 Striated muscles occur 

 in the intestines of the Tench, a fresh-water fish of the carp family. 

 Du Bois-Raymond is the highest authority on the muscles and his ex- 

 periments are the oftenest quoted. Experiments with muscles detached 

 from the body are commonly made with those of the frog, generally 

 the gastrocnemius or calf muscle of the leg. The frog being cold 

 blooded his muscles can be kept in the normal state longer than those 

 of a warm blooded animal. 



The muscles of mammals detached from the body lose their 

 power of contraction in from 20 to 30 minutes. The muscles of frogs, 

 however, retain their contractile power in some cases as long as forty- 

 eight hours in an ordinary temperature. At a temperature of to 1 C 

 (near freezing point) the same muscle might retain its contractility for 

 eight days. At a temperature of 45 C, 113 F, however, the contractile 

 power is lost in a few minutes. ( Rosenthal. ) Fresh muscle from the 

 frog does not shorten spontaneously, but will contract when irritated, as 

 by pinching with tweezers, or smearing with strong acid, or by electri- 

 city, &c. These are called irritants. 

 1 Rosenthal Muscles & Nerves, 101. 



