276 PHYSIOLOGY 



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 depen- 

 dent for its activity on the central nervous system. Cut off from 

 this it is flabby and motionless. Its sole function is to contract 

 efficiently and smartly on receipt of impulses arriving along its nerve. 

 It is only necessary therefore that these impulses should be of one 

 character motor, and we know that each fibre of a muscle, such as 



FIG. 98. Tracing from the retractor penis muscle of the dog, showing 

 lengthening (inhibition) on stimulation of the nervus erigens, and a 

 smart contraction on stimulating the pudic (motor) nerve. (Move- 

 ments of muscle reduced |.) 



the 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 inner va- 

 tion may be at one time in a state of relaxation, at another in a state 

 of tonic contraction. In order that the central nervous system 

 should have efficient control over such a tissue, it must be able to 

 influence it in two directions : it must be able to induce a contraction 

 or increase a contraction already present, and it must also be able to 

 put an end to a spontaneous contraction, i.e. to induce relaxation. In 

 order to carry out these two effects, smooth muscle receives nerve 

 fibres of two kinds from the central nervous system, one kind motor, 

 analogous to the motor nerves of skeletal muscle, the other kind 

 inhibitory, causing relaxation or cessation of a previous contraction. 

 All these fibres belong to the visceral or ' autonomic ' system. They are 

 connected with ganglion- cells in their course outside the central nervous 

 system, and their ultimate ramifications in the muscle are always non- 

 medullated. A typical tracing of the opposite effects of these two sets 

 of nerves is given in Fig. 98. 



