THE LIMBS OF THE HORSE 33 



that in the dissection of the limbs of the horse one is dealing with 

 an extreme departure from the typical mammalian architecture. 



Before proceeding to remove the skin, the dissector should examine 

 the surface of the forearm. In the anterior, lateral, and posterior 

 regions the bones are buried under muscular masses, which present 

 considerable resistance to palpation owing to the stoutness of their 

 fascial investment. Of bony prominences, the most obvious are those 

 produced by the olecranon of the ulna and the projection at the 

 posterior and lateral part of the carpus, due to the accessory carpal 

 (pisiform) bone. The medial border of the radius can be followed 

 throughout the whole of its length. Of the lateral border, however, 

 only the distal portion can be felt through the skin. The position of 

 the rest of the border is indicated by a shallow groove, which becomes 

 deeper towards the carpus, where it is complicated by a ridge formed 

 by the edge of the extensor digiti quinti muscle. 



The outstanding rounded mass in front of the forearm is formed 

 by the fleshy portion of the radial extensor of the carpus and the 

 common extensor of the digits.^ 



The resisting muscular mass at the back of the forearm is formed 

 by flexors and one extensor. 



A flattened, oval, horny callosity, popularly known as the " chestnut," 

 occurs on the skin in the distal third of the medial face of the fore- 

 arm. The'^prominence and size of the callosity is subject to much 

 variation, and is related to the " fineness " of the skin over the body 

 generally. In the average animal its greatest (vertical) diameter 

 is about 50 mm. The exact location of the callosity is also variable ; 

 and, while one can say that it lies over the radial flexor of the carpus, 

 it is impossible to give any useful indication respecting its distance 

 from the radiocarpal joint. A similar callosity occurs in the pelvic 

 limb in connection with the skin of the medial surface of the tarsus. 



Conflicting views are held respecting the homology of these callosi- 

 ties. Authorities have regarded them variously as representing the 

 rudiment of a digit, modified skin glands such as are present in the 

 carpal region of certain mammals, degenerate carpal sense-organs, and 

 even as altered foot-pads. It is safest to conclude that the question 

 is still open. 



Dissection. — The skin must now be removed from the entire forearm 

 and carpus, and from the proximal part of the metacarpus. In doing 



' Though the horse possesses but one functional digit, the plural is used here 

 and throughout in order that it may be clear that the muscle (in the main) 

 corresponds to the common extensor of other mammals. 



