CHAP, i.] THE SPINAL CORD. 923 



In other words, the muscle in the living body possesses a latent 

 tendency to shorten, which is continually being counteracted by 

 its disposition and attachments. In studying muscular contraction 

 we saw ( 87) 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 occurrence 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 corre- 

 spondingly 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 property 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 

 molecular 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 ( 83), 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 ( 549) 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 experience, 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 movements. This resistance, which is quite indepen- 

 dent of, that is to say, which may be clearly recognized in the 



