34 OSTEOLOGY. 



The superior or proximal extremity consists of a head, a neck, 

 and a tubercle. The head, which rests in a cavity in the verte- 

 brae, is separated by a vertical furrow, in which the interarticular 

 ligament is attached, into two convex articular surfaces one of 

 which is directed forwards and inwards, and the other backwards 

 and inwards. The neck is the constricted portion supporting the 

 head; it is strong and roughened for ligamentous attachment, 

 and has a groove for the intercostal nerves and blood-vessels 

 running across it in front. The tubercle is the prominent emi- 

 nence at the posterior part of the neck ; it has a flat, smooth 

 facet, which articulates posteriorly with the transverse process of 

 a vertebra. External to the tubercle is another eminence, the 

 angle, where the rib is suddenly bent downwards; the distance 

 between the angle and the head of the rib gradually increases 

 from the third to the last, the first two scarcely possessing 

 angles. In the third and fourth ribs the angles and the tuber- 

 cles are from one to two inches apart, which distance increases, 

 in a well-formed horse, to nearly five inches in the last. In horses 

 possessing well-rounded chests the costal angles are plainly marked 

 and separated from the vertebras, so as to form large costal grooves 

 for the dorsal muscles ; in flat-sided animals they are indistinct 

 and closer to the spinal column, the grooves being consequently 

 narrower. 



The inferior or distal extremity is expanded and constructed 

 of more elastic bone than the proximal extremity ; it is quite 

 porous, and joined firmly to its cartilage. The anterior border 

 is rough, and excavated along its upper third to give attachment 

 to the intercostal muscles. The posterior border is grooved along 

 its upper third for the intercostal muscles, nerves, and blood- 

 vessels ; in both borders the distal portion is thinner in the anterior 

 than in the posterior ribs. The external surface is convex in 

 every direction ; between the head and the angle it is roughened 

 for muscular attachment, and below this point smooth, excepting 

 in a few places the chief of which, a short distance below the 

 angle, give attachment to muscles. The internal surface, concave 

 from above downwards, and convex from side to side, is smooth, 

 and for the greater part covered by the pleura, the membrane 

 lining the thorax. 



With the exception of the last, and sometimes the last two on 

 each side, each rib has its cartilage. These cartilages, which 

 contain a large quantity of earthy or bony material, pass obliquely 



