THE COSTAL CARTILAGES — THE STERNUM 45 



Variations.— A nineteenth rib on one side or both is not rare. It is usually imperfectly 

 developed and quite variable. In some cases it is a mere strip of cartilage connected by ligament 

 with the first lumbar transverse process; in other cases it is ossified, and may be fused with the 

 process; in others again it is connected with an additional presacral vertebra which may be 

 thoracic or lumbar or ambiguous in character. It is commonly floating, but may be attached to 

 the eighteenth. Reduction in number is uncommon. Fusion of adjacent ribs sometimes occurs. 



THE COSTAL CARTILAGES 



The first costal cartilage is an inch or more (2.5 to 3 cm.) in length. The upper 

 part is very wide and thick. The sternal end is small. The two articulate with 

 each other as well as with the sternum. The cartilages of the other sternal ribs 

 increase progressively in length and become more rounded. The sternal end is 

 expanded and has an elliptical convex facet for articulation with the sternum. The 

 cartilages of the asternal ribs are long, slender, and pointed. The ninth is the 



II' - Cariniform cartilage 



Ventral border -^■■1^' 



Costal cartilages -.- II '^' ^^^^^ 



I 

 I 

 I 



Xiphoid cartilage 



Fig. 23. — Sterxum and Cost.\l Cartilages of Horse, Ventral View, (.\fter Ellenberger-Baum, Anat. f. 



Kiinstler.) 



longest, and is very firmly attached to the eighth; behind this they diminish pro- 

 gressively in size, and are attached to each other by elastic tissue. Except in the 

 case of the first, the cartilage does not continue the direction of the rib, but forms 

 with the latter an angle which is open in front, and decreases in obtuseness from 

 second to last. 



More or less extensive ossification is to be regarded as a normal occurrence, 

 especially in the cartilages of the sternal ribs. 



The Sternum 



The sternum of the horse is shaped somewhat like a canoe; it is compressed 

 laterally, except in its posterior part, which is flattened dorso-ventrally. It is 

 inclined obHquely so that the posterior end is about six to eight inches (15 to 20 

 cm.) lower than the anterior. 



The dorsal surface has the form of a very narrow isosceles triangle with the 

 apex in front. It is concave longitucUnally, flattened transversely. 



The lateral surfaces are convex above, slightly concave below, and diminish 

 in extent behind. Each presents on its upper part seven articular cavities (Fovese 

 costales), which receive the sternal ends of the second to the eighth costal cartilages 

 inclusive. These cavities are situated in series at the intersternebral junctions. 



