POSTERIOR LIMB. 21 



sents an external and an internal tuberosity. Its articular sur- 

 face consists of two shallow cavities and a median ridge, all three 

 of which extend diagonally forward and backward. 



This bone articulates with four others; viz., femur, patella, 

 fibula, and one of the tarsal bones. 



The fibula is a small rudimentary bone, which articulates with 

 the external portion of the head of the tibia, and is situated on 

 the external side of that bone. 



The patella is small, short and located in front of the lower 

 extremity of the femur, and attached to the tibia below by three 

 strong ligaments. It is displaced outward when the limb is in 

 the condition popularly known as "stifled." This bone offers 

 for study three faces : superior, to which muscles are attached ; 

 the anterior, which is convex and somewhat irregular: and the 

 posterior, which is so shaped as to fit nicely into the femoral 

 groove already mentioned. 



The foot bears a very close resemblance in its anatomy to the 

 corresponding portion of the anterior limb, and is divided for 

 study into the tarsus, metatarsus, and digit. 



The tarsus is composed of six or seven small bones, arranged 

 in two rows, the upper row containing the astragalus and os cal- 

 cis. The lower row usually consists of four small bones. Two 

 of these are especially interesting because they are the common 

 seat of bone spavin. 



The astragalus is interesting because it furnishes the articular 

 surfaces for the lower end of the tibia. 



The os calcis is somewhat elongated and lies behind the as- 

 tragalus. It corresponds to the prominent portion of the human 

 heel. The bones of this region correspond to the human tarsal 

 bones making up the ankle. 



The metatarsus consists of three bones, a large one in the 

 center and one small rudimentary bone on each side; i. e., on 

 the inside and outside of the middle bone. The large metacarpal 

 furnishes the supporting axis for this region as in the anterior 

 limb. The shaft is fairly smooth and has a small articular sur- 

 face on each side for the rudimentary metatarsal. It is supposed 

 that the small metacarpal and metatarsal bones are merely sur- 

 vivals in the process of evolution and that in the earlier history 

 of the various animals from which the modern horse is descended 



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