66 PHYSIOLOGY OF MUSCLE AND NERVE 



tissue are capable of withstanding, is astonishingly great. To begin 

 with, therefore, the successive application of these weights gives rise 

 to a curve, the concavity of which is turned toward the abscissa, 

 while eventually, when the elasticity of the muscle has been overcome, 

 it is turned downward. Dead muscle is less extensible than living 

 muscle, whereas contracted or fatigued muscles are more extensible. 



The elastic power of muscle tissue serves as a protection against 

 injury by sudden counter forces. Especially in the case of the striated 

 type, it minimizes the possibility of damage to the bones and tendons. 

 Furthermore, this elastic tension prevents the muscles from relaxing 

 completely so that they are always held in a condition of "setting" 

 which enables them to react more promptly as well as more smoothly. 

 It serves, therefore, to conserve the energy which is required to produce 

 a contraction. In many cases, the skeletal muscles are arranged 

 antagonistically to one another, so that the contraction of one set places 

 the others under a certain elastic tension. This is especially true of the 

 flexors and extensors of the arms. Elastic forces also play a most 

 important part in the production of the pressure which is required to 

 drive the blood through the circulatory system. In this particular 

 instance, however, this function is assigned to the elastic tissue of the 

 blood-vessels rather than to the smooth muscle cells. Cardiac muscle 

 exhibits its elastic power most clearly at the beginning of ventricular 

 systole, i.e., directly after the ventricular wall has been fully distended 

 by the forcible emptying of the auricles. 



Tonicity of Muscle. A normal muscle, when resting, is not re- 

 tained in a condition of complete relaxation, but is held in a state of the 

 slightest possible contraction. The factor which is chiefly responsible 

 for this tonic setting of a muscle is the elastic tension of its constituents. 

 Thus we find that the division of one sciatic nerve causes the cor- 

 responding leg to hang down much lower than that of the opposite side, 

 because its muscles have now entered a state of complete relaxation. 

 It should be noted, however, that the tension of the muscles does not 

 constitute the condition of tonus, but is merely one of the prerequisites 

 thereof. Tonus in reality is the result of a continuous influx of im- 

 pulses from the central nervous system. 



In further analysis of this phenomenon it will be found that 

 ganglion cells and their efferent adjuncts retain their function only 

 if allowed to remain in contact with those sense organs which keep 

 them in activity by means of their centripetal impulses. If these 

 impulses are prevented from reaching the center, the corresponding 

 effector becomes inactive and loses its tonus. So it is with muscle. 

 It cannot be said, therefore, that the cells of the spinal cord are auto- 

 matically concerned with the production of tonus, because their 

 activity, and hence also the tonus of the muscles innervated by them, 

 disappears very promptly after the dorsal roots of the spinal nerves 

 have been divided. It will be remembered that these paths serve as 

 highways for a large number of afferent impulses. Their destruction, 



