TH-E ANTEBIOB LIMBS. 85 



plantar fissures These originate at the root of the basilar process, are 

 directed obliquely downwards and inwards, and open into the plantar fora- 

 mina, the external orifices of two large canals which enter the bone and unite 

 in its interior to form the semilunar sinus. 



Borders.— The superior describes a curve, with the convexity forward 

 and presents : 1, In its middle, the pijramidal eminence of the os pedis a 

 single triangular process, flat before and behind, roughened on its anterior 

 aspect, and concurring, by its posterior surface, to form the articular surface 

 which responds to that of the second phalanx , 2, Laterally, two facets of 

 insertion which encroach on the anterior surface, and even advance, 

 posteriorly, nearly to the preplantar fissure. The inferior border is thin^ 

 dentated, convex, and semicircular; it is perforated by from five to ten 

 large foramina which pass into the bone. The posterior border is slightly 

 concave ; on it is observed a very narrow, transversely elongated, diarthrodial 

 facet, which becomes confounded with the superior large articular surface, 

 and is adapted to a similar facet on the navicular bone. 



Lateral angles. — These are two projections directed backwards, on whose 

 summit the three borders of the bone unite, and which gives attachment 

 to the latei.il fibro-cartilages. A deep notch, the origin of the preplantar' 

 fissure, separates each into two particular eminences : one, the superior, 

 nanied by M. Bouley the basilar process ; the other, the inferior, prolonged 

 behind, and designated by Bracy Clark the retrossal process, from retro, 

 behind, and o»ga, bone. 



Structure. — The os pedis exhibits in its interior the semilunar sinus, a 

 cylindrical, transversely elongated, and semicircular cavity resulting from 

 the arching anastomoses of the two plantar canals. From this cavity pass 

 off numerous channels, which anastomose frequently with each other, and 

 open externally by the foramina on the anterior face of the bone, or by 

 thosg on its inferior border. The os pedis has for its base a nucleus of 

 spongy substance, surrounded by a layer of compact tissue. The latter is 

 thicker towards the pyramidal eminence than elsewhere, and sends into 

 the interior numerous prolongations which form the walls of the semilunar 

 sinus, as well as the bony channels which spring from it. 



Development. — The third phalanx, formed from a single nucleus of 

 ossification, 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 very 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. 



The complementary fihro-cartilaginous apparatus of the os pedis. — To 

 understand 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 should have been obtained ; therefore a detailed description will only 

 be given when the Horse's foot is studied as a whole. It will be suflScient 

 here to state that this apparatus consists of two lateral pieces, the fibro- 

 cartilages of the OS pedis, united behind and below by the plantar cushion, a 

 fibrous and elastic mass on which rests the navicular bone through the 

 medium of the perforans tendon. 

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