554 THE AETERLES. 



keratogenous apparatus, and terminates by anastomosing with a branch of 

 the artery now to be noticed. 



6. The coronary circle, 1 formed by two transverse branches one anterior, 

 the other posterior, springing at a right angle from the digital artery, under 

 the cartilaginous plate of the os pedis passes around the coronary bone to 

 meet the analogous branches of the opposite artery, to anastomose with them 

 directly and by inosculation. The coronary circle therefore presents two 

 distinct portions : one posterior, placed above the superior border of the small 

 sesamoid, beneath the perforans tendon ; the other anterior, more extensive 

 and voluminous, covered on the sides by the lateral cartilages of the foot, 

 and in its front or middle part by the expansion of the anterior extensor 

 tendon of the phalanges. 



The collateral ramuscules furnished by the posterior part of the circle 

 are small, few, and of no interest. 



Among the branches arising from the anterior portion, there is only a 

 single pair of arteries to be noted, which are remarkable for their mode 

 of distribution and their volume. They originate near the border of 

 the extensor tendon, and immediately divide into two divergent branches : 

 one the internal, which passes across that tendon to anastomose with the 

 homologous branch of the opposite side ; the other, external, passes backward 

 to meet the cutigeral branch furnished by the artery of the plantar 

 cushion, and joins that vessel. From this disposition results a very fine 

 superficial vascular arch around the coronet, which is well named the 

 circumflex artery of the coronary cushion ; it is situated a little above the 

 cutidural artery, beneath the skin of the coronet, and looks as if incrusted 

 in that membrane ; by its two extremities it rests on the arteries of the 

 plantar cushion, and is fed by the two principal vessels of the coronary 

 circle ; while it furnishes ascending anastomosing ramuscules to the inferior 

 divisions of the perpendicular artery, as well as numerous descending 

 branches passing into the coronary cushion and the laminal tissue of the 

 foot. 



Such is the ordinary disposition of the coronary circle and its super- 

 ficial arch the circumflex artery of the coronary substance ; though it varies 

 much in different animals, and even in the feet of the same animal. To attempt 

 to describe here the variations we have seen would be supererogatory, and we 

 may limit ourselves to saying that these varieties were almost exclusively 

 confined to the origin of the branches composing these two circular vessels 

 and their manner of arrangement, without modifying in any way the general 

 disposition of the circles. 2 



Terminal divisions. These are, as has been already mentioned, the plantar 

 and preplantar ungual arteries* 



a. The preplantar ungual artery is the smallest of these two terminal 

 branches. Situated at first inside the basilar process of the third phalanx, 

 it turns round this to traverse the notch which separates this process from 

 the retrossal eminence, is lodged with a satellite nerve in the preplantar 



1 So named because it ncircles the coronet. 



2 We may notice here one of these variations, which is somewhat frequently met 

 with in the anterior limb. This consists in the anterior descending branch of the 

 perpendicular artery uniting at its terminal extremity with the circumflex artery of the 

 coronHry substance, which it concurs to form. 



3 In all treatises on anatomy these vessels are simply designated the plantar and 

 preplantar arteries. We have added the epithet ungueal to distinguish these arteries 

 from the properly so-caile.l plantar branches the terminal divisions of the posterior 

 tibial artery. 



