CHAP. ii. HEREDITARY DISEASE IN HORSES. 411 



bone of the hind limb. The bone which projects backward on the 

 outside is the calcaneum, below it is the cuboid. On the inner side of 

 the lower part of the calcaneum, and partly in front of the latter, is the 

 astragalus which presents upwards a beautiful pulley-like surface to 

 form a hinge joint with the lower end of the leg-bone (the tibia). 

 Below the astragalus is a flat, slightly curved bone, the true navicular 

 of anatomists, but endowed with several other names by veterinarians 

 (e.g., scaphoid, cuneiform magnum, etc.). Below the true navicular 

 again are two other bones, the cuneiforms (external and internal), 

 resting upon the cannon-bone. In looking, from the front, at the 

 hock of the off hind leg of the horse, the following arrangement shows 

 approximately the relative positions of the six bones, though the 

 internal cuneiform (or cuneiform parvum) is so far behind that it 

 would not be visible : 



Z ASTRAGALUS. 



< 



O 



< NAVICULAR. 



O 



CUBOID. EXT.C. INT.C. 



It will be seen that the bones of the hock are, as it were, "two 

 deep," on the outside, and " three deep " on the inside, regarded from 

 above to below. Spavin is the name given to any exostosis, or bony 

 outgrowth, in the region of the hock. It is usually situated at the 

 inner and lower part of the region. Between and towards the front of 

 the cuneiform bones is a common seat of the disorder. When spavins 

 occur on the external faces of the bones they may easily be felt. If 

 they occur between the hock-bones, however, there is little or no 

 external indication, and the disorder is then known as occult spavin. 



Cataract is an affection of the eye. This delicate and beautiful 

 organ is constructed on practically the same principle as a photo- 

 grapher's camera, for it contains, a short distance behind the pupil, a 

 double-convex mass of transparent gelatinous material called the 

 crystalline lens, the function of which is to focus images of external 

 objects on a screen at the back of the eye. Any interference with the 

 proper working of the crystalline lens results necessarily in imperfect 

 vision, and, it may be, in blindness. The seat of true cataract is in 

 this lens, or in the capsule which encloses it. 



In order to adequately understand the changes accompanying 

 diseases, it is obvious that some preliminary knowledge of the anatomy 

 and physiology of the organs involved is desirable. In other words, 

 it is necessary to know the normal structure and functions of organs in 

 health before proceeding to study them when diseased. 



It will be convenient here to indicate a few of the more important 

 anatomical characters in which the ox, sheep, and pig differ from the 

 horse. 



