THE PHENOMENON OF CONTRACTION. 51 



supposed to furnish the energy for the shortening are relatively- 

 inconspicuous when measured by the heat produced or the 

 oxygen consumed. Corresponding to this small consumption of 

 material and energy these contractions show very little fatigue- 

 effect. The muscles of the limbs and the trunk exhibit this con- 

 dition of tonus under ordinary conditions, and it may be shown 

 that it depends upon stimuli of some kind coming to the muscle 

 from the central nervous system. The tonus, to use a technical 

 term, is neurogenic rather than mijogenic, that is to say, the cause 

 of it hes in the nervous system and not in the muscle itself. If 

 the nerve going to the muscle is cut the muscle loses its tone, be- 

 coming longer and more flaccid. Moreover, the tonus is a reflex 

 phenomenon. The nerve centers do not send out their impulses 

 by some form of automatic activity, but they are stimulated to 

 action b}^ sensory or afferent impulses coming into the center along 

 afferent nerve fibers. If the afferent paths from the leg of a frog 

 are destroyed without injuring the motor paths, as may be done by 

 cutting the posterior roots of the spinal nerves supplying the leg, 

 the muscles lose their tone. When the frog is suspended the leg on 

 the operated side hangs lower than the other owing to the fact that 

 in it the normally acting tonus of the flexor muscles is removed. 

 Sherrington* has shown that this reflex tonus is especially con- 

 nected with the maintenance of posture. In the standing position, 

 for example, the necessary degree of fixation of the joints is pro- 

 vided for by tonic contraction of the surrounding muscles. In such 

 cases the sensory stimuli which are responsible for the excitation of 

 the tonus are assumed to arise in the muscle itself or in the ac- 

 cessory tendons and joints. The reflex belongs to the so-called 

 proprioceptive group. (See p. 276.) It must be recognized, how- 

 ever, that tonus maybe excited reflexly through other afferent paths. 

 Emotional and intellectual activity, that is to say, impulses de- 

 scending from the higher centers, have such an effect on the 

 musculature. External cold by its action on the cutaneous nerves 

 throws the muscles reflexly into a state of increased tonus. The 

 sensory apparatus in the labyrinth of the internal ear seems to 

 have special adaptive relations to the tonus of the muscles main- 

 taining the posture of the head, and so on. It is impossible 

 at present to give a satisfactory explanation of the difference 

 between an ordinary contraction of a muscle and the tonus con- 

 traction. While each is dependent upon stimuli received through 

 the nerve fibers, the underlying chemical and physical changes 

 must be different or must affect different structures. It has been 

 suggested (Botazzi f) , for example, that the ordinary contraction is 



* Sherrington, "Brain," 38, 235, 1915. 



t Botazzi, "Journal of Physiology," 21, 1, 1897. 



