THE RIBS 



43 



cave, and situated almost directly outward from the cavity for the head of the rib ; 

 traced backward it becomes smaller and flatter, and gradually comes to lie behind 

 the cavity for the head of the rib, with which it is fused on the last and often also on 

 the next to the last thoracic vertebra. The processes in the lumbar region have a 

 characteristic elongated plate-like form. In the sacral region they are fused to 

 form the wings and lateral parts of the sacrum. In the coccygeal region they are 

 at first of considerable size relatively, but undergo rapid reduction, and disappear 

 at the fifth or sixth vertebra. 



The cavities for the heads of the ribs diminish progressively in size and depth 

 from fir.st to last. 



The mammillary processes are usually distinct on the fourteenth to the seven- 

 teenth thoracic vertebrae. In front of these they blend with the transverse, behind 

 with the anterior articular processes. 



The length of the vertebral column (including the intervertebral fibro-cartilages) in a horse 

 of medium size is about 260 to 265 cm. (ca. S feet 8 inches to 8 feet 10 inches). The relative lengths 

 of the variou.s regions appear to vary most in the neck and back. 



The table below gives the measurements in centimeters in a trotting stallion of medium size 

 and in an adult Percheron stallion. The percentages are in round numbers. 



The Ribs 



There are usually eighteen pairs of ribs in the horse, but a nineteenth rib on 

 one side or both is not at all rare. Eight are sternal or "true" ribs, the remainder 

 asternal or "false." Ribs from different parts of the series vary much in length, 

 curvature, and other characters. We will therefore consider as a type a rib from 

 the middle of the series first, and afterward note the chief serial differences A 

 tjrpical rib has the following characters: 



The shaft or body (Corpus costse) is elongated, relativel}^ very narrow, and 

 strongly curved; the curvature is most pronounced in the dorsal third, and the 

 ventral part is twisted and inclined inward, so that when a rib is laid with its outer 

 surface on the table, the ventral end is raised. The external surface is convex in 

 its length and also transversely; its anterior part is, however, grooved longitudin- 

 ally. A distinct angle, i. e., a point at which the curve of the rib changes rather 

 suddenly, as in man, can scarcely be said to exist in the horse. The term is often 

 applied, however, to a corresponding rough elevation which gives attachment to the 

 iho-costalis muscle ; it is most distinct on the fourth to the eighth inclusive. The 

 internal surface is smooth, concave in its length, and rounded from side to side; 

 the costal groove, situated posteriorly, is very cUstinct above and fades out about 

 the middle. The anterior border is concave, the posterior convex. 



The vertebral extremity (Extremitas vertebralis) consists of the head, neck, 

 and tubercle. The head (Capitulum costse) has an articular surface (Facies 

 articularis capituli costse), composed of two convex facets, anterior and posterior, 

 separated by a groove for the attachment of the conjugal ligament. It articulates 

 with the cavity formed by facets on the bodies of two adjacent thoracic vertebrae 



