OTHER FORMS OF CONTRACTILE TISSUE 247 



Indeed by clamping two curarised sartorius muscles together, as in the 

 diagram (Fig. 96), it is found that stimulation of the muscle A causes con- 

 traction of the muscle B. The current of action of A in this case 

 has served to excite a contraction in B. 



It must be remembered that in all unstriated muscle the fibres are sur- 

 rounded by a network of non-medullated nerve fibres. Some physiologists are 

 inclined to ascribe to these fibres an important part in the propagation of the 

 contraction wave. In the case of the heart muscle however, it can be shown 

 almost conclusively that the propagation takes place independently of nerve FIG. 96. 

 fibres, and probably the same is true for many kinds of involuntary muscle. 



INFLUENCE OF TEMPERATURE. Smooth muscle is extremely sus- 

 ceptible to changes of temperature ; as a rule warming causes relaxation, 

 while 'application of cold causes a tonic contraction. The condition of the 

 muscle at any given time depends not only on its actual temperature, 

 but also on the rapidity with which this temperature has been reached. 

 Thus a rapid cooling of the retractor penis muscle of a dog from 35 to 25 

 may cause a contraction as extensive as would be produced by a slow cooling 

 to 5C. On warming a muscle from 30 to 50C. it lengthens gradually up 

 to about 40, and it may then undergo a marked heat contraction (varying 

 in degree in different muscles) at about 50C., which may pass off at a 

 somewhat higher temperature. It is killed somewhere between 40 and 

 50C. It seems very doubtful whether any true rigor mortis occurs in 

 smooth muscle. The hard contracted appearance of the smooth muscle 

 in a recently dead animal is chiefly conditioned by the fall of temperature. 

 On excising the muscle and warming it up to body temperature it may 

 again relax and show signs of irritability two or three days after the death 



of the animal. Different smooth 

 muscles however vary very much 

 in their tenacity of life. 



DOUBLE INNERVATION. 

 Voluntary muscle is absolutely 

 dependent for its activity on the 

 central nervous system. Cut off 

 from^this it is flabby and motion- 

 less. Its sole function is to con- 

 tract efficiently and smartly on re- 

 ceipt of impulses arriving along its 

 nerve. It is only necessary therefore 



FIG. 97. Tracing from the retractor penis muscle that these impulses should be of one 

 of the dog, showing lengthening (inhibition) i -, , 



on stimulation of the nSrvus erigens, and a character motor,andweknowthat 

 smart contraction on stimulating the pudic each fibre of a muscle, such as the 

 (motor) nerve. (Movements of muscle re- . 



duced . ) sartorius, receives one efferent nerve 



fibre terminating in an end-plate. 



In the case of smooth muscle we have a tissue which has an activity 

 and reactive power of its own, and apart from its innervation may be at 

 one time in a state of relaxation, at another in a state of tonic contraction, 



