580 THE SPINAL COED. 



The skeletal muscles have been described as being placed " on the stretch " 

 in. the living body. If a muscle be cut away from its attachments at each 

 end, it shortens ; if it be cut across, it gapes. 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 mus- 

 cular contraction we saw ( 85) that the shortening of a contraction is fol- 

 lowed 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 correspond- 

 ingly 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 nat- 

 ural 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 a skel- 

 etal 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 ( 81), 

 that the irritability of a muscle is markedly affected by the section of its 

 nerve, i. e. t by severance from the central nervous system ; and again ( 462), 

 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, shows that the nu- 

 tritive 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- 

 ments. This resistance, which is quite independent of, that is to say, which 

 may be clearly recognized in the absence of all distinct muscular contrac- 

 tions of volitional or other origin, is an expression of muscular tone, of the 

 effort of the various muscles to maintain their "natural" length. In many 

 cases of disease this resistance is felt to be obviously less than normal ; the 

 limb is spoken of as " limp " or " flabby," or as having " a want of tone." In 

 other cases of disease, on the other hand, this resistance is markedly increased ; 

 the limb is felt to be stiff or rigid ; more or less force is needed to change it 

 from a flexed to an extended, or from an extended to a flexed condition ; and 

 in the range of disease we may meet with very varying amounts of increased 

 resistance, from a condition which is only slightly above the normal to one 

 of extreme rigidity. In some cases the condition of the muscle is such as at 



