POSTERIOR MEMBER. 239 



muscles have to displace the body-weight as well as to overcome the 

 inertia, while, in the second case, they only raise the member and pro- 

 ject it forward. The energy and the extent of their contraction will 

 determine the intensity and amplitude of the femoral force which, in 

 concert with the stifle and the hock, communicates to the trunk the 

 initial impulsion, the action, as it is customary to say. 



Direction. — The direction of the thigh cannot be suitably de- 

 scribed unless we understand \vell the signification which it is neces- 

 sary to accord to this word. 



In animal mechanics, osseous segments have an axis of form which is not 

 always their axis of movement. The latter being defined as the imaginary line 

 which connects the two probable centres of movement, it is clear that it will 

 differ from the axis of form whenever the articular surfaces are situated in front, 

 behind, without, or within the axis of the latter. This has already been noticed 

 in the case of the humerus, and is evident here again in the consideration of the 

 femur. The axis of form of this bone follows almost exactly the direction of 

 a line connecting the trochanter with the fossa which exists between the trochlea 

 and the external condyle ; the axis of movement, on the contrary, joins the 

 centre of the coxo-femoral to the centre of the femoro-tibial articulation, and 

 crosses the first by reason of the fact that the head of the femur occupies the 

 internal side of this bone instead of being situated directly at its superior 

 extremity. 



In spite of the difficulties which, in the living animal, hinder the determina- 

 tion of this fact, we may obtain the result in an approximate manner by seeking 

 the two points indicative of the two aforesaid articular centres ; these are, on the 

 one part, the concavity of the trochanter, and, on the other, the middle of the 

 length of the external femoro-tibial ligament. The line joining these two 

 points will constitute the axis of movement of the femur. In many subjects, the 

 horse being supposed to be in equilibrium, it is almost vertical ; in others, it falls 

 to a slight extent obliquely forward and downward ; finally, there are some in 

 which it is oblique in an inverse sense, — that is to say, downward and backward. 



The direction of the thigh should satisfy the four principal require- 

 ments which follow : 



1st. Give to the coxo-femoral angle, already reduced by the hori- 

 zontal direction of the croup, a sufficiently wide opening. 



2d. Permit of an extensive separation of the branches of the 

 femoro-tibial angle, while allowing, at the same time, a feeble obliquity 

 of the leg. 



3d. Not alter the vertical axis, which implies the tangency of the 

 hock to the vertical line falling from the point of the buttock. 



4th. Finally, maintain the stifle in a certain state of separation 

 from the median plane. 



We estimate, from our researches, that a mean inclination of 80 

 degrees fulfils all these desiderata in rapid motors. The obliquity in 



