SUPERIOR FACE OF THE BODY. 127 



point of the buttock, and makes the croup long or short. This is 

 illustrated in Fig. 37. The lines AB and A'B' are of unequal 

 lengths, whilst they support angles whose sides are exactly equal ; 

 nothing but the inclination of these lines has changed. It follows from 

 this that if the increase of the length of the croup can exist without 

 influence upon the real length of the ilium and ischium, it modifies, 

 nevertheless, the direction of the region in such a way that it tends 

 to become, in general, more horizontal. We will return to this subject 

 hereafter. 



It is curious to observe that in our larger quadrupeds the line of 

 direction of the croup always passes perceptibly above the centre of 

 the coxo-femoral articulation, whilst in our smaller species, as rodents 

 and carnivora, for example, the ilium and the ischium standing more 

 nearly in the same line, and making a less angle the one with the other, 

 this line passes almost through the centre of the joint. The ilio-ischial 

 angle, in this respect, seems more marked, or, if it be preferred, the angle 

 is the more effaced as the volume of the body becomes less and the 

 nimbleness of the movements greater. Thus it augments in size grad- 

 ually from the horse and the ox to the pig ; from the pig to the smaller 

 ruminants ; and, finally, from the latter to the carnivora and the rodents. 

 This relative openness, or largeness of the angle, ^vith the corresponding 

 modifications in the length of the muscles, and of their incidence with 

 the osseous levers, and the play of the angles of locomotion in the pos- 

 terior extremity, seems specially adapted to the execution of the gallop 

 and leaping. 



Whatever may be the interpretation, it is well known that in 

 horses of great speed this angle is more open than in draught-horses. 

 In animals used for rapid services, this conformation has the preference. 

 It is manifested externally by the relation of the centre of the articu- 

 lation with the line of direction of the croup. 



The length of the croup, however, is due, more than to aught else, to 

 that of the ilium and the ischium, the ilio-ischial angle remaining the 

 same. These offer some important individual differences. Thus we 



have seen the relation of these two bones ^-^. — vary from 1.55 to 1.87. 



ischium " 



The ratio augments in fast-trotting horses, and diminishes in hunters, 

 saddle-horses, and draught-horses. The length of the ilium determines 

 that of the muscles attaching to it, and favors the amplitude of the 

 femoral extension, whilst that of the ischium fixes the vertebral column 

 when the flexors act, and facilitates the rotation of the pelvis downward 

 and backward. 



