256 THE ADRENAL SYSTEM AND VASOMOTOR FUNCTIONS. 



regards contraction a surplus, not of fuel, but of the active 

 element which underlies the effects of "draught" in the engine: 

 i.e., oxygen, that contained in the "oxidizing" substance. The 

 fuel myosinogen is always present in our muscles when 

 they are normal, and the activity of the combustion is regu- 

 lated there, as elsewhere in Nature, by the quantity of oxygen 

 admitted. Yet, how is the conscious or unconscious control 

 implied by the word "voluntary" carried out? 



The muscular arteriole during complete muscular retrac- 

 tion is only just sufficiently patent to allow the passage of 

 enough blood-plasma containing the oxidizing substance to 

 sustain the nutrition of the muscular tissues, and other proc- 

 esses through which their functional efficiency is insured. But 

 the fact must not be overlooked, as emphasized by Foster, that 

 the relaxation is an essential part of the whole act; indeed, 

 no less important than the shortening itself. Again, a com- 

 pletely retracted muscle is not a relaxed muscle; it is precisely 

 in the opposite state, i.e., in a condition of tension between 

 its insertions, and if either one of the latter- be cut the mus- 

 cle recedes toward the other. This feature is well exemplified 

 after amputations. The biceps can contract unimpeded, for 

 example, three times the extent that its skeletal attachments 

 will normally allow; fractures of the olecranon or of the 

 patella are familiar examples, notwithstanding the fact that 

 the muscles thus liberated at one end are held partly retracted 

 by the surrounding structures. Indeed, a normal muscle can 

 aptly be compared to a piece of rubber stretched between two 

 fixed points, and contraction really represents a relaxation of 

 the stress. But there are variations in the resistance to which 

 this stress is submitted, and it is here that the identity of the 

 controlling concept appears as an independent factor, while 

 that of the motor mechanism also emphasizes itself by phe- 

 nomena that cannot logically be considered as elements of the 

 process through which the "voluntary" impulse is transmitted, 

 nor of the transmitting organs, the motor nerves. 



If the arm is flexed, say, at an angle of 90 degrees, it can 

 be held in this position without fatigue for some time. But 

 if a sufficiently heavy weight be placed in the hand, the arm 

 remaining in precisely the same position, marked evidences of 



