TEE ANTERIOR LIMBS. 



117 



the walls of the semilunar sinus, as well as the bony channels which spring 

 from it. 



Development. — The third phalanx, formed from a sin^^le nucleus of ossifica- 

 tion, undergoes numerous changes in its configuration during life. Thus, in the 

 young animal the lateral angles are thick, obtuse, and but little prolonged 

 posteriorly ; but as it grows older, they increase in length and become salient. 

 The development they then assume, is due to the progressive ossification of the 

 lateral cartilages implanted on their surface. It often happens, in \ery old horses, 

 that this ossifying process is carried to an extreme degree, and nearly the whole 

 substance of these complementary organs is invaded. From the commencement, 

 its inevitable result is to convert the notch which separates the basilar from the 

 retrossal process into a foramen. 



T/i£ complfmcntarij Jibro-cartilat/iuoifs apparatus of the os pedis. — To under- 

 stand properly the disposition of this portion of the foot, it is necessary that a 

 previous knowledge of the ligaments 



and tendons attached to the os pedis Fig. 75. 



should have been accpiired ; therefore • a. 3, 



a detailed description will be given 

 when the Horse's foot is studied as a 

 whole, in the article on the Sense of 

 Touch. It will be sufficient here to state 

 that this apparatus consists of two 

 lateral pieces — the fihro-cartiIa//es of the 

 OS pedis, united behind and below by 

 the plantar rushion — a fibrous and elastic 

 mass on which rests the navicular bone, 

 through the medium of the perforans 

 tendon. 



The Small Sesamoid or Navicular 

 Bone (Figs. 72, TSV — This short bone 

 is annexed to the third phalanx, behind 

 which it is situated ; it is elongated 

 transversely, flattened above and below, 

 and narrow at its extremities. It offers : 

 1. A superior face, on which are pro- 

 longed the glenoid cavities and the 

 median ridge of the articular surface 

 of the OS pedis ; it articulates with the 



second phalanx. 2. An inferior face, divided by a slight ridge into two undulated 

 facets, and covered with cartilage to form a gliding surface. An anterior border, 

 channeled lengthways by a groove of insertion, above which is remarked a diar- 

 throdial facet that brings the small sesamoid into contact with the posterior border 

 of the third phalanx. 4. A posterior border and two extremities, for ligamentous 

 insertion. This bone, as well as the sesamoids, originates from a single centre of 

 ossification. It is formed of a layer of compact tissue enveloping /a nucleus of 

 very condensed spongy substance. (For differences in the Ass, see Posterior 

 Limb.) 



NAVICDLAR BONE OF THE HORSE. 



Anterior Border and Inferior Face. 1, articu- 

 lar facet for the facet on the posterior border 

 of the OS pedis; 2, roughened groove on the 

 anterior border ; 3, inferior face, smooth and 

 undulated. B, Posterior Border and Superior 

 Face. 1, Articular face for the lower end of 

 the second ]ihalanx; 2, posterior border, with 

 many foramina. 



