22 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



of time. It has been shown that the extensibility of a muscle is 

 greater in the contracted than in the resting state. 



The curve of extension described above for skeletal muscle 

 holds also for so-called plain muscle. This latter tissue forms a 

 portion of the walls of the various viscera, the stomach, bladder, 

 uterus, blood-vessels, etc., and the facts shown by the above curve 

 enter frequently into the explanation of the physical phenomena 

 exhibited by the viscera. For instance, it follows from this curve 

 that the force of the heart beat will cause less expansion in an 

 artery already distended by a high blood-pressure than in one in 

 which the blood-pressure is lower. 



The Irritability and Contractility of Muscle. Under normal 

 conditions in the body a muscle is made to contract by a stimulus 

 received from the central nervous system through its motor nerve. 

 If the latter is severed the muscle is paralyzed. We owe to Haller, 

 the great physiologist of the eighteenth century, the proof that 

 a muscle thus isolated can still be made to contract by an artificial 

 stimulus e. g., an electrical shock applied directly to it. This 

 significant discovery removed from physiology the old and harmful 

 idea of animal spirits, which were supposed to be generated in the 

 central nervous system and to cause the swelling of a muscle during 

 contraction by flowing to it along the connecting nerve. But to 

 remove a muscle from the body and make it contract by an artificial 

 stimulus does not prove that the muscle substance itself is capable 

 of being acted upon by the stimulus, since in such an experiment 

 the endings of the nerve in the muscle are still intact, and it may 

 be that the stimulus acts only on them and thus affects the muscle 

 indirectly. In a number of ways, however, physiologists have 

 found that the muscle substance can be made to contract by a 

 stimulus applied directly to it, and therefore exhibits what is 

 known as independent irritability. The term irritability, according 

 to modern usage, means that a tissue can be made to exhibit its 

 peculiar form of functional activity when stimulated, e. g., a 

 muscle cell will contract, a gland cell will secrete, etc., and inde- 

 pendent irritability in the case under consideration means simply 

 that the muscle gives its reaction of contraction when artificial 

 stimuli are applied directly to its substance. This conception 

 of irritability was first introduced by Francis Glisson (1597-1677), 

 a celebrated English physician.* Subsequent writers frequently 

 used the term as synonymous with contractility and as applicable 

 only to the muscle. But it is now used for all living tissues in 

 the sense here indicated. A simple proof of the independent 

 irritability of a striated muscle is obtained by cutting the motor 

 nerve going to it and stimulating the muscle after several days. 

 * See Foster's " History of Physiology," p. 287. 



