PHYSIOLOGICAL PROPERTIES OF THE MUSCLES. 535 



nerves. In the cramps of cholera, tetanus, or the convulsions from strychnine, these 

 distressing sensations are very marked. The so-called recurrent sensibility of the anterior 

 roots of the spinal nerves is probably due in part to the tetanic contractions produced by 

 galvanizing these filaments. This question, however, will be taken up again in connection 

 with the nervous system. 



If the muscles possess any general sensibility, it is very faint. A muscle may be 

 lacerated or irritated in any way without producing actual pain, although we always can 

 appreciate the contraction produced by irritants and the sense of tension when the mus- 

 cles are drawn upon. 



Muscular Contractility, or Irritability. Physiologists now regard muscular irrita- 

 bility as synonymous with contractility; and, perhaps, the latter term more nearly 

 expresses the fact, although the term irritability, applied to the nerves, and even of late 

 years to the glands, is one very generally used. 



By irritability we understand a property belonging to highly-organized parts, which 

 enables them to perform certain peculiar and characteristic functions in obedience to a 

 proper stimulus. In the sense in which the term is generally received, it is proper to apply 

 it to any tissue or organ that performs its vital function, so called, under a natural or an 

 artificial stimulus. The nerves receive impressions and carry a stimulus to the muscles, 

 causing them to contract. This property, which is always present during life, under normal 

 conditions, and which persists for a certain period after death, is called nervous irritability. 

 It has lately been shown that the application of a proper stimulus will induce secretion by 

 the glands ; and Bernard has called this glandular irritability. The application of a stim- 

 ulus to the muscular tissue causes the fibres to contract ; and this is muscular irritability. 

 As it always involves contraction and is extinct only when the muscles can no longer act, 

 it is equally proper to call this property contractility. No property, such as we under- 

 stand by this definition of irritability, is manifested by tissues or organs that have purely 

 passive or mechanical functions, such as bones, cartilages, and fibrous or elastic mem- 

 branes. The term irritability can only be applied properly to nerves or nerve-centres, 

 to contractile structures, and to glands. 



During life and under normal conditions, the muscles will always contract in obe- 

 dience to a proper stimulus applied either directly or through the nerves. In the natural 

 action of the organism, this contraction is induced by nervous influence through reflex 

 action or volition. Still, a muscle may be living and yet have lost its contractility. 

 For example, after a muscle has been for a long time paralyzed and disused, the applica- 

 tion of the most powerful galvanic excitation will fail to induce contraction. But, when 

 we examine such a muscle with the microscope, it is found that the nutrition has become 

 profoundly affected, and that the contractile substance has disappeared, giving place to 

 inert fatty matter. Muscular contractility persists for a certain time after death and in 

 muscles separated from the body; and this fact has been taken advantage of by physiolo- 

 gists in the study of the so-called vital properties of the muscular tissue. We have 

 already seen that a muscle detached from the living body continues for a time to respire, 

 and probably it undergoes some of the changes of disassimilation observed in the organ- 

 ism. So long as these changes are restricted to the limits of physical and chemical integ- 

 rity of the fibre, contractility remains. As these processes are very slow in the cold- 

 blooded animals, the irritability of all the parts persists for a considerable time after 

 death. We have repeatedly demonstrated muscular contractility, several days after 

 death, in alligators and turtles. 



In the human subject and the warm-blooded animals, the muscles cease to respond to 

 excitation a few hours after death, although the time of disappearance of irritability is 

 very variable. ISTysten, in a number of experiments upon the disappearance of contrac- 

 tility in the human subject after decapitation, found that different parts lost their con- 

 tractility at different periods, but that generally this depended upon exposure to the air. 



