THE RIBS 45 



reduced and much closer together in the back, larger and interlocking in the lumbar 

 region. 



The transverse processes are large and outstanding in the neck, where they 

 form the lateral boundary of a ventral groove occupied by the longus colli muscle. 

 In the back they are short and stout, and are characterized by the facets for the 

 tubercles of the ribs. On the first thoracic vertebra this facet is large, deeply con- 

 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 first to last. 



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

 teenth thoracic vertebraj. 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 nine feet (ca. 2.7 meters). The relative lengths of the various regions ap- 

 pear to vary most in the neck and back. The following average lengths of the several regions 

 were obtained by measurement of several subjects: Cervical, 70 cm.; thoracic, 86 cm.; lumbar, 

 34 cm.; sacral, 20 cm.; coccygeal, 60 cm. The percentage values are approximately 26, 32, 12.5 

 7..5, 22. 



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 ribs, the remainder asternal. 

 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; such a rib has the following 

 characters: , 



The shaft or body (Corpus costae) is elongated, relatively 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 lateral 

 surface on the table, the sternal end is raised. The lateral 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 some- 

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

 to the longissimus muscle. The medial surface is smooth, concave in its length, 

 and rounded from side to side; the costal groove (Sulcus costs;) , situated posteriorly, 

 is very distinct above and fades out about the middle; it contains the intercostal 

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



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

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

 articularis capituU costs) which is composed of two convex facets, anterior and 

 posterior, separated by a groove (Sulcus capituU) for the attachment of the con- 

 jugal Ugament. It articulates with the cavity formed by facets on the bodies of 

 two adjacent thoracic vertebrae and the intervertebral fibro-cartilage. The neck 

 (Collum costse) is roughened above and in front. The tubercle (Tuberculum costs) 

 is placed above and behind the junction of neck and shaft; it has an oval surface 



