206 ANIMAL PHYSIOLOGY. 
The great differences known to exist between individuals of 
the same species in strength, endurance, fleetness, and other 
particulars in which the muscles are concerned, raise numer- 
ous interesting inquiries. The build of the greyhound or race- 
horse suggests in itself part of the explanation on mechanical 
principles, lung capacity, etc. But when it is found that one. 
dog, horse, deer, or man excels another of the same race in 
swiftness or endurance, and there is nothing in the form to 
furnish a solution, we are prompted to ask whether the muscles 
may not contract more energetically, experience a shortening 
of the latent period, or other phase of contraction; or whether 
they produce less of waste-products or get rid of them more 
rapidly. The whole subject is extremely complicated, and we 
may say here that there is some evidence to show that in races 
of dogs and other animals which surpass their fellows, the 
nerve regulating the heart and lungs (vagus) has greater power ; 
but, leaving this and much more out of the account, it is likely 
there are individual differences in the functional nature of the 
muscle. Of equal or more importance is the energizing influ- 
ence of the nervous system, which probably under great excite- 
ment (public boat-races, etc.) acts to produce in man those 
supermaximal contractions which seem to leave the muscle 
long the worse of its unusual action. The nerve-centers, it is 
likely, suffer still more from excessive discharge of nerve-force 
(as we may speak of it for the present) necessary to originate 
the muscular work. Hence the importance of training to 
minimize the non-effective expenditure, ascertain the capacity 
possessed, learn the direction in which weaknesses lie; and 
equally important the much-neglected period of rest before 
actual contests—if such are to be undertaken at all—so that 
all the activities of the body may gather head, and thus be 
prepared to meet the unusual demand upon them. 
The law of rhythm in organic nature is beautifully illus- 
trated by the behavior of nerve and especially muscle; at least 
it is more obvious in the case of muscle, at this stage of our 
progress. , 
The regularity with which one phase succeeds another in a 
single contraction; the essentially rhythmic (vibratory) char- 
acter of tetanus, fatigue and recovery; the recurrence of in- 
crease and decrease in the muscle and nerve currents—in fact, 
the whole history of muscle is an admirable commentary on 
the truth of the law of rhythm, into which in further detail 
space will not permit us to enter. 
