66 HUMAN PHYSIOLOGY 



the beating heart. The current of action in the second 

 muscle in each case may stimulate the first. (Practical 

 Physiology, Chap. VIII.) 



YII. Extensibility. 



The extensibility of muscle is increased during contraction 

 so that the application of a weight causes a greater lengthen- 

 ing than when the muscle is at test. 



Visceral Muscles. 



In several important respects the visceral muscles differ in 

 their mode of action from the skeletal muscles. 



1. Their connection with nerves is by no means so definite 

 and precise, for, instead of each nerve-fibre ending in a 

 muscle- fibre, the nerves to non- striped muscle form an 

 irregular network upon them, and the muscle-fibre appears 

 to be capable of action, both before these nerves have 

 developed in the embryo and when the influence of these 

 nerves has been cut off in the adult. In the intestine the 

 mode of action of the muscles is largely dominated by the 

 plexus of nerves (see p. 328). 



2. The great features of the action of visceral muscle are 

 1st, its tendency to sustained tonic contraction; and 2nd, 

 its spontaneous regular rhythmic contraction and relaxation. 



1st. The continuous slight tonic contraction is seen in all 

 the visceral muscles, and, while it may be increased or 

 diminished by the intervention of nerves, it appears to be 

 largely independent and an expression of the continuous 

 metabolism of the muscle protoplasm. 



2raZ. The rhythmic contractions and relaxations are not 

 equally manifest in all situations, nor are they so continuous, 

 but they are well marked in the muscles around such hollow 

 viscera as the intestines, bladder, and uterus. Like the tonic 

 contractions, they are to a certain extent independent of 

 nerve action, but are influenced by it. 



These contractions recur at regular intervals of varying 

 duration. Each contraction lasts for a considerable period 

 sometimes over a minute and the relaxation is correspond- 

 ingly long. Everything which increases the rate of chemical 



