714 TONE OF SKELETAL MUSCLES. [BOOK in. 



gapes. In other words, the muscle in the living body possesses 

 a latent tendency to shorten, which is continually being counter- 

 acted by its disposition and attachments. In studying muscular 

 contraction we saw ( 81) that the shortening of a contraction 

 is followed by a relaxation or return to the former length, both 

 the contraction and relaxation being the result of molecular 

 changes in the living muscular substance. We have now to 

 extend our view and to recognize that, apart from the occur- 

 rence of ordinary contractions, molecular changes are by 

 means of nutritive processes continually going on in the muscle 

 in such a way that the muscle, though continually on the stretch, 

 does not permanently lengthen, but retains the power to shorten 

 upon removal or lessening of the stretch, and conversely though 

 possessing this power of shortening permits itself to lengthen 

 when the stretch is increased. In this way the muscle is able 

 to accommodate itself to variations in the amount of stretch to 

 which it is from time to time subjected. When a flexor muscle 

 for instance contracts, the antagonistic extensor muscle is put on 

 an increased stretch and is correspondingly lengthened ; when 

 the contraction of the flexor passes off the extensor returns to 

 its previous length ; and so in other instances. Thus by virtue 

 of certain changes within itself a muscle maintains what may 

 be called its natural length in -the body, always returning to 

 that natural length both after being shortened and after being 

 stretched. In this the muscle does no more than do the other 

 tissues of the body which, within limits, retain their natural 

 form under the varied stress and strain of life ; but the prop- 

 erty is conspicuous in the muscle ; and its effects in skeletal 

 muscles correspond so closely to those of arterial tone, that we 

 may venture to speak of it as skeletal tone. Indeed, the molec- 

 ular changes at the bottom of both are probably the same. 



These changes are an expression of the life of the muscle; 

 they disappear when the muscle dies and enters into rigor mortis; 

 and moreover, during life they vary in intensity so that the 

 ' tone ' varies in amount according to the nutritive changes 

 going on. We have seen reason to believe that the nutrition of 

 a muscle as of other tissues is governed in some way by the 

 central nervous system. We saw, in treating of muscle and 

 nerve ( 78), that the irritability of a muscle is markedly 

 affected by the section of its nerve, i.e. by severance from the 

 central nervous system; and again ( 439) in speaking of the 

 so-called trophic action of the nervous system, we referred to 

 changes in the nutrition of muscles occasioned by diseases of 

 the nervous system. And experience, especially clinical experi- 

 ence, shews that the nutritive changes which determine tone are 

 very closely dependent on a due action of the central nervous 

 system. When we handle the limb of a healthy man, we find 

 that it offers a certain amount of resistance to passive move- 



