THE POSTERIOR LIMBS. 



139 



Fig. 91. 



the condyles. It is slightly oblique downwards and inwards, and appears to 

 continue in front the intercondyloid notch. Of the two lips which border its 

 cavity laterally, the internal is the thickest and the most prominent. Between 

 the external and the corresponding condyle, is seen a digital fossa for muscular 

 insertion. 



Structure and development. — The femur, very spongy at its extremities, is 

 developed from four principal centres of ossification : one for the body, another 

 for the articular head, the third for the trochanter, and the last for the inferior 

 extremity alone. 



The femur of the Ass offers several differential characters, the principal of 

 which have reference to the length of the neck, the development of the third 

 trochanter (trochanter minor), and the curvatures of the 

 diaphysis. The greater length of the neck causes the 

 internal trochanter to be some distance from the hori- 

 zontal plane on which the inner face of the bone lies ; 

 in the Horse this trochanter is always in contact with 

 the plane. The small trochanter is less developed than 

 in the Horse, as may be seen on laying the bone on its 

 external border ; for the femur of the Ass rests by the 

 trochanter major and external condyle, while in the Horse, 

 it lies on the latter and the subtrochanteric crest. With 

 regard to the curvatures, there is remarked a slight 

 diminution in that which carries the head of the bone 

 backwards, and a slight increase in the twist of the 

 diaphysis around its longitudinal axis, which alters the 

 equilibrium of the bon* so that it is impossible to have 

 it in stable equilibrium when it rests on the trochanter 

 major, head, and inner lip of the trochlea ; this is easily 

 accomplished with the femur of the Horse. 



By the dimensions of the neck and internal trochanter, 

 the femur of the Hinny and Mule holds a middle place 

 between their progenitors ; while in the development of 

 the trochanter major, and the degree of torsion around 

 its longitudinal axis, the femur of the Hinny much resembles that of the Ass, 

 and the Mule that of the Horse, though the conditions of equilibrium always 

 remain the same as in the latter. 



SECTION OF LEFT FEMUR, 

 SHOWING ITS STRUCTURE. 



Differential Characters in the Thigh-bone of the other Animals. 



In all the domesticated animals except Solipeds, the femur tends to become curved longi- 

 tudinally, prismatic, and triangular; the posterior face contracts, and the surfaces for insertion 

 that it presents gradually approach each other, until they become confounded, and form a 

 linea aspera in certain species. The head is more distinct ; the internal trochanter is a rough 

 tubercle, and is joined to the large trochanter by an oblique ridge ; the large trochanter sub- 

 sides, and forms a single mass, the summit and convexity of which are confounded ; the third 

 trochanter, the fossa, and the supra-condyloid crest are more or less effaced. In addition to 

 these modifications, there are others special to each species. 



A. Rvuninants.— In the Ox, there is no subtrochanteric crest ; the supra-condyloid fossa 

 is shallow, and the crest little noticeable. The head is well detached, and has its centre 

 excavated by a shallow fossa of insertion. The trochlea is narrow, and its inner border ascends 

 much higher on the anterior face of the bone than the external. 



In the Sheep and Goat, the general form of the femur resembles that of the Ox. It is 

 observed, however, that the body is slightly curved backwards ; that the supra-condyloid fossa 



