06 HANDBOOK OF PHYSIOLOGY. 



supply of arterial blood to them is kept up. Thus, after ligature of the 

 main arterial trunk of a limb, the power of moving the muscles is par- 

 tially or wholly lost, until the collateral circulation is established; and 

 when, in animals, the abdominal aorta is tied, the hind legs are ren- 

 dered almost powerless. 



The same fact may be readily shown by compressing the abdominal 

 aorta in a rabbit for about 10 minutes; if the pressure be released and 

 the animal be placed on the ground, it will work itself along with its 

 front legs, while the hind legs sprawl helplessly behind. Gradually the 

 muscles recover their power and become quite as efficient as before. 



So, also, it is to the imperfect supply of arterial blood to the muscular 

 tissue of the heart that the cessation of the action of this organ in as- 

 phyxia is in some measure due. 



Fatigue. The irritability of muscle is decreased by undue functional 

 activity. The cause of the diminished irritability is twofold when a 

 muscle contracts, part of its substance is expended, part of its store of 

 nutriment is exhausted, and it cannot readily contract again until the 

 loss is made up. To this extent fatigue is much the same in its effect 

 as cutting off or diminishing the blood-supply. The other cause for the 

 diminution of irritability is the accumulation of poisonous products in 

 the lymphatics of the muscle substances generated during contraction. 



Separation from Central Nervous System. Generally a muscle begins 

 to lose its irritability to all forms of stimuli about two weeks after its 

 nerve is severed. Within a short time, however, its readiness of re- 

 sponse to mechanical stimuli and to direct battery currents is height- 

 ened, while to induction shocks it is lessened. The increase of irrita- 

 bility reaches its maximum in about seven weeks, after which the 

 irritability to all forms of stimuli diminishes, until it is completely lost 

 toward the end of the seventh or eighth month. 



The loss of irritability in muscle is due to degenerative changes in 

 its protoplasm. But the cause of the degeneration is a matter of con- 

 troversy, being considered due to loss of trophic influences from the 

 central nervous system on the one hand, and to circulatory disturbances 

 on the other. 



Use. Not only irritability but strength and power of endurance in 

 muscle are increased by use. The effect of properly regulated exercises 

 on muscles is too well known to need more than bare mention. And, 

 on the contrary, 



Disuse leads to diminution or loss of irritability. This fact is famil- 

 iarly shown when a limb is disabled for a time, as through breaking a 

 bone, in the stiffness of the muscles and the slowness with which they 

 respond to the will. 



