-GO OUTLINES OF EQUINE ANATOMY. 



under it are two smooth grooves for tlie passage of the 

 axillary artery, and (below it) the axillary vein. The rest 

 of this surface is occupied by the attachuient of serratus 

 magnus. The cartilage of the first rib is the shortest and 

 stoutest, and very closely approximates its fellow of the 

 ■opposite side. 



The last rib is the smallest, and its ])osterior margi7i 

 gives attachment to quadratus lumborum. To the inner 

 surface of the last two ribs the psoas magnus attached. 

 We have seen that the anterior ribs are strongest, broadest, 

 and most firmly connected together ; they are thus admir- 

 ably adapted to the formation of the thorax, which not 

 only serves to protect important viscera, but also affords 

 an extensive base for attachment of the fore limbs to the 

 trunk. This cavity is completed below by the STERNUM, 

 which consists of "^the haemal spines of six vertebral seg- 

 ments embedded in a cartilaginous mass, so that they may 

 be viewed as one bone. Each of these bones presents 

 seven surfaces. The superior surface is smooth, forms the 

 floor of the thorax, where it gives attachment to ligaments, 

 and to the sterno-costales interni. That of the sixth bone 

 on either side presents the articulatory surfaces for the 

 cartilages of the last two true ribs, of the fifth bone for 

 the sixth sterno-costal cartilage. ThQ anterior im^ posterior 

 surfaces are rough for attachment of the cartilage, while 

 the superior and inferior lateral surfaces, scarcely distin- 

 guishable in the first bone, assist in forming the lateral 

 and inferior surfaces of the bone taken as a whole. These 

 bones decrease in depth and increase in width from before 

 backwards. They are prolonged anteriorly and posteriorly 

 by cartilages. The anterior cartilage i^resents a rounded 

 supero-anterior extremity from which a short concave 

 superior margin runs to the antero- superior part of the 

 first bone. An inferior margin convex, rounded smooth 

 along the central line of the under surface, blending with 

 the inferior margins of the bone almost as far backwards 

 as the posterior cartilage. It is flattened from side to side, 

 though growing broader posteriorly, and thus it gives 

 the sternum a keel-shape ; it is the cariniform cartilage. 

 Its prominent antero-superior part would form the prow. 

 The ensiform, posterior or xiphoid cartilage is a mem- 

 braniform piece of cartilage, flattened from above down- 

 wards, widest at its attachment to the posterior part of 



