STERNUM. 35 



forwards and inwards ; they diminish in thickness from before 

 laackwards, but increase in length to ^he eleventh or twelfth, 

 ■after which they again grow shorter. Those attached to the 

 true ribs are smallest at their proximal ends, becoming expanded 

 before they articulate with the sternum ; those of the false ribs 

 are largest at their proximal ends, and, gradually tapering to fine 

 points below, they pass forwards and downwards, so that, having 

 no inferior articulations, each is overlapped by the succeeding 

 one. 



The special ribs are the two first and the last four or five. 

 The first is shorter and straighter than the others, with rough 

 surfaces, rounded borders, and no well-defined posterior groove ; 

 its head is irregular in shape, and exceeded in size by the tubercle, 

 and its distal end is flattened and expanded laterally, joining 

 the short thick cartilage, which contains much ossific matter. 

 The second possesses the above-named characters of the first in a 

 less degree. The last four or five are slender ; the articular 

 surfaces of their tubercles and the posterior divisions of their 

 beads are usually continuous ; their curvature is great ; they are 

 broadest immediately behind the tubercle, and their cartilages 

 are remarkably slender and elongated. 



Each true rib has four articular surfaces, two on the head, 

 ■one on the tubercle, and one at the distal end. 



STERNUM. 



The sternum or breast-bone, a comparatively small and short 

 bone in- the horse, is single in the adult, and placed in the 

 inferior longitudinal line of the body at the antero-inferior part 

 of the thorax. The anterior portion somewhat resembles the 

 keel and cut- water of a boat ; it is elongated and concave above, 

 convex below, with its anterior part flattened laterally, and its 

 posterior part flattened above and below. It is constructed of 

 six, rarely seven, irregularly-formed segments or sternebrcB, 

 united by cartilage in the young animal, and by partial ossifica- 

 •^os in the adult ; complete ossification seldom or never takes 

 place in this bone. Its anterior end is surmounted by the carini- 

 form- cartilage, and its posterior extremity prolonged by the 

 «nsiform or xiphoid cartilage. 



Its superior surface is triangular in outline and concave, the 

 superior sternal ligaments traversing its lateral borders. Its 



