THE HIND LIMB 83 



We have already pointed out that while the fore-limbs are united to the 

 trunk by muscles alone the hind ones have the additional connection pro- 

 vided by the hip-joint, where the head of the femur (fig. 66) fits into the 

 acetahuhim or cup -like cavity formed by the bones of the pelvis. By 

 this arrangement the hind extremities are brought to act directly on the 

 hiiunch, and through it on the spine, so that as the feet grip the ground 

 and the limbs straighten out by eftacement of the articular angles, the body 

 is forced forward in a succession of propulsive efforts which the muscles of 

 the haunch impart to it. The various regions composing the hind ex- 

 tremity have their analogues in the fore one, but it is noticeable that the 

 order of direction in the one is reversed in the other — at least so far as the 

 upper segments are concerned. Thus, that portion of the pelvis which 

 slopes downward and backward is the analogue of the shoulder, which 

 slopes downward and forward. 



The tliigh, which passes downward and forward, is the counterpart of the 

 humerus or upper arm. which sIo^jcs downward and backward. 



The tibia or second thioh, whose inclination is downward and back- 

 ward, is the equivalent of the radius or forearm, which is straight. The 

 parts beneath these are more or less uniform in direction in the two members. 

 It will thus be seen that in both the fore and the hind extremities the 

 ossific segments are so disposed as to form a series of angles which are more 

 or less considerable in proportion to the length and slope of the bony 

 levers. 



In the act of progression these angles are alternately closed and opened, 

 resulting in the consecutive shortening and lengthening of the limbs. On 

 the extent to which these movements are capable of being effected will 

 depend the length of the stride and the force of the impulse which the 

 hind-limbs are capable of imparting to the body. In this connection it is 

 oliserved that in speedy animals the angles below that of the shoulder 

 in front and the pelvis behind are remarkable for their open condition, 

 hence the peculiar straightness of the legs of the race-horse. 



The greater ol)liquity of the shoulder and the more horizontal position 

 of the croup in horses of speed tend to diminish the scapulo-humural and 

 coxo-femoral angles, and, as Goubaux and Barrier observe, " facilitate the 

 forward and backward movement of the inferior regions, and give them the 

 freedom of extending themselves effectively to pass over the ground or 

 communicate the impulsion". In horses of draught, where the steps are 

 short, extreme lengthening of the locomotory column, so necessary to 

 animals of speed, is not required. The shoulder therefore is less sloping, 

 the croup more inclined, and the upper angles consequently more open. 

 The lower angles of the limbs, which, as we have seen, are open in horses of 



