414 THE EXTERIOR OF THE HORSE. 



in fact, that work increases in propoftion to the mass, whilst it aug- 

 ments as the square of the velocity. This is equivalent to saying that 

 if the mass be increased 2, 3, 4, 5 times, the work is rendered 2, 3, 4, 5 

 times greater, whilst if the velocity be modified in the same degree, 

 the motor would accomplish a work 4, 9, 16, and 25 times greater; an 

 important idea, already foreshadowing the fact that an animal employed 

 at speed will be able to endure the frequent repetition of the efFort only 

 during a relatively limited period of time. 



Fatigue. — On the other hand, what is fatigue f Littre ^ defines 

 it a painful sensation accompanied by a difficulty of exertion, and 

 caused by excessive work. Now, as this feeling is manifested by a 

 cessation of locomotory activity, which, as is known, is the immediate 

 consequence of the contraction of the muscles, it follows that to obtain 

 a correct conception of the nature of fatigue we must previously recall, 

 in a few words, the physiological conditions of muscular contraction. 



Muscular Contraction. — The fundamental property of a muscle, as we 

 have seen, is the power of contracting under the influence of certain stimuli. In 

 the organism, it is the nervous system which presides over every contraction ; but 

 its role is facilitated or retarded according to different conditions. Thus, it is 

 proved that contractility is increased by an active circulation of the blood, by 

 the presence of oxygen, a moderate temperature, a proper rest, etc. It is slack- 

 ened or diminished by diametrically opposite causes, an arrest of circulation, 

 carbonic acid, lactic acid, cold, prolonged inactivity, excessive functional activity, 

 etc. During the physiological contraction the nervous impulses are communi- 

 cated to a muscle interruptedly, but, at the same time, to all its elements, in 

 such a manner that the organ is shortened proportionally to its length, and with 

 a power which depends upon the number of its primitive fibres. But under the 

 influence of fatigue the contraction becomes slower and less energetic. Similar 

 effects are produced by cold and all interferences with a free circulation. Whilst 

 the muscle is contracted it becomes acid, consumes oxygen, hydrocarbon prin- 

 ciples, and, in default of these, albuminous substances ; its circulation increases, 

 and the venous blood is much darker. Its chemical composition is also very 

 different from that of the inactive muscle. There are formed carbonic acid, lactic 

 acid, urea, creatine, as well as other azotic and non-azotic waste products ; all of 

 them results either of oxidation, or of a particular fermentation, or, finally, of 

 a pure and simple disassociation of the elements which the muscle had assimi- 

 lated and which constitute the complete aliment. 



However this may be, the products of respiration and muscular contraction 

 have to be reabsorbed by the veins and the lymphatics, without which the organ 

 becomes overloaded and loses its contractility. 



Muscular contraction, to be produced, requires, then, a supply of blood and 

 an excitation ; to be continued, it implies a free, active circulation, and a suffi- 

 cient amount of the exciting principle. As the proportion of waste is propor- 

 tionate to the rapidity, the intensity, the duration, and the repetition of the con- 



1 LiUr6, Dictionnaire de la langue fran^aise. 



