POSTERIOR MEMBER. 281 



fetlock : it is that of the metacarpo- or metatarso-phalangeal sheath, also called the 

 great sesamoid sheath. They both facilitate the gliding of the flexor tendons of 

 the phalanges, the first against the posterior face of the hock or the knee, and 

 the second over the angle of the fetlock ; both permit, besides, the movements of 

 the two cords upon each other. 



Finally, against the posterior face of the principal bone of the canon, in a 

 sort of gutter formed by the splint bones, is lodged the vast ligamentous brace 

 of the fetlock, a (Fig. 93), which is bifid inferiorly and terminates upon the 

 sesamoid bones. 



It is not without interest to note, in passing, a somewhat infrequent fact of 

 which science, nevertheless, has a certain number of examples ; we refer to the 

 existence of snpernmnerary digits ' on the internal face of the canon of one, two, 

 or all four of the members of the horse at one time. This anomaly consists in the 

 abnormal and almost complete development of the internal digit, which, in ordi- 

 nary conditions, is simply represented by the corresponding but aborted meta- 

 tarsal or metacarpal bone. 



The external digit occurs much more rarely. Professor R. S. Huidekoper 

 has observed a very remarkable case in a Texas horse. The animal had, in all, 

 ten hoofs, three on each anterior member and two on each posterior. The an- 

 terior supernumerary digits were formed each of three phalanges ; their hoofs 

 descended almost to the ground. The supernumerary digits on the posterior 

 members existed only on the internal side ; their phalanges were rudimentary 

 and their horny covering only reached to the middle of the pastern.'^ 



Role and Action of the Canon. — The canon is a locomotory 

 lever which plays a most important role in progression, station, and 

 impulsion. Let us examine it rapidly from these diverse relations. 



It is primarily the seat of two principal movements, ^ca;/on and 

 &)ctension. 



The first, much more pronounced in the anterior member, elevates 

 the foot and places the canon in a favorable position to accomplish the 

 second, which it is desirable should be as extensive as possible. The 

 length of the forearm and that of the tibia are, as we know, in relation 

 with the amplitude of these displacements. 



The movement of the canon in the t-s^o members is of an inverse 

 order, on account of the opposite positions of the articular angles. 

 When that in the anterior member is extended, it simply passes over 

 the ground, places itself in prolongation with the radius, arrives thus 

 on the soil, and preserves this attitude as long as the foot remains on 

 the ground. In the posterior member, its extension commences a little 

 before the foot is in station, and continues during the whole period that 



1 Supernumerary digits constitute simply an example of the law of reversion to the penta- 

 dactylous or five-toed type of the foot of the ancestral forms of the horse, as the eo-hippus, in 

 which four digits exist. (Harger.) 



* R. Huidekoper, Note communiqu^e. 



