498 Dynamic Theory. 



a current instead of a contraction. As an illustration let us see how a 

 boy dodges a snow-ball. First, he sees the ball coming. This means 

 that the light reflected from the ball, decomposes some of the tissues of 

 his retina and produces electric tension there, which sends a current to 

 the brain. Dewar has demonstrated that light falling upon the eye has 

 this effect of setting up an electric current in the optic nerve. There is 

 in the brain the same state of things that is found in the muscle. The 

 brain is permeated by fine blood vessels holding oxygen, waiting to be 

 united to the tissue composing the brain. The current from the eye up 

 the optic nerve, furnishes the necessary force to -enable this union to 

 take place. And this union, as in the case of the muscle, develops 

 electrical tension, which instantly relieves itself in a current to certain 

 muscles. Here a new tension is created, followed by contraction, which 

 qiiickty throws the body or head of the Tx)y to one side, and the ball is 

 dodged. 



The average force of the muscle contractions is probably from six to 

 ten times as powerful as the force of the nerve currents which set them 

 off. It is ascertained that about one-fifth of the blood of the body goes 

 to supply the brain. A considerable part of this is consumed in the 

 production of brain changes which do not result in movement of mus- 

 cles, but in thought, emotion, &c. The other four- fifths supplies the 

 work of digestion, secretion and muscle contraction. The lifting power 

 of the muscle of the fore-arm in man, is at the rate of about 100 pounds 

 per square inch cross section of muscle fibre. For the muscles of the 

 feet, calf, &c. , the power is considerably less. Of course the contrac- 

 tions of the muscle are made at the expense of its own tissues, some of 

 them being used to create the electricity by which the contraction is 

 produced. 



A muscle out of the body will recover from exhaustion partly, but not 

 completely, of course. Each stimulation uses up the power of recovery 

 till it is soon lost. In the living bod}^ however, this power of recovery 

 is continuous while nourishment is supplied to the muscle in the blood, 

 till it diminishes b}^ the wearing out of the system in age or disease. 

 During health, exhaustion and recovery of muscle adds to its power, so 

 that it may recover more than it loses. ( Rosenthal 80. ) It is in conse- 

 quence of this extremely important fact that exercise of an}^ part 

 strengthens and adds to it. There are many cases in which bodies en- 

 ter into combination with other bodies with much more energy at the 

 instant they are disengaged from former combinations. At this instant 

 the}?- are said to be in a nascent state, that is, just born. Exercise, by 

 reducing some of the constituents to this fresh condition, allows of this 

 more complete recombination, and hence an improved condition. 



The muscle fibre terminates at each end by a perfect disc, which is 



