THE THORAX. 67 



cavities, which receive the inferior extremity of the cartilages of the true 

 ribs. These cavities are elongated vertically, and draw closer to each other 

 as they extend backwards. The inferior part, which is more extensive before 

 than behind, oifers to the powerful pectoral muscles a large surface for 

 insertion. 



Borders. — The two lateral borders separate the superior from the lateral 

 faces ; they are situated above the diarthrodial cavities, are united anteriorly, 

 and each gives attachment to a fibrous band. The inferior border is opposite 

 the superior face ; convex, thin, and very prominent in its anterior two-thirds, 

 it somewhat resembles the keel of a ship. 



Extremities. — The anterior flattened on each side and curved upwards, 

 exceeds to some extent the first articular cavity of the lateral faces, and in 

 this way constitutes the cervical prolongation of the sternum. The posterior 

 extremity is flattened superiorly and inferiorly, and forms a large cartila- 

 ginous plate, very thin, concave above, convex below, which has received the 

 name of the abdominal prolongation (ensiform cartilage) or xiphoid appendage. 



Structure and development. — The sternum is one of the parts of the skeleton 

 which do not submit to complete osseous transformation. It is developed, 

 in solipeds, from six single centres of spongy substance, ranged one behind 

 the other, like beads on a string. Those centres never coalesce to form a 

 solid piece, but remain separated, during the life of the animal, by the 

 primitive cartilaginous mass. The latter constitutes the entire anterior pro- 

 longation of the bone and its carina, as well as the xiphoid appendage. When 

 these parts of the sternum become ossified, which is rare, it is only partially. 



2. The Bibs. 



As has been already noticed, on each side of the thorax eignteen ribs are 

 counted. These are nearly parallel to each other, and separated by the 

 intervals termed the intercostal spaces. Attached by their superior extremity 

 to the vertebrae of the dorsal region, these bones terminate at their inferior 

 extremity by an elastic and flexible prolongation, named the costal cartilage, 

 by means of which they are brought into direct or indirect relations with 

 the sternum. The characters common to all the ribs will be first noticed, 

 then the special features which serve to distinguish them from each other, 

 and, lastly, the differences they exhibit in other than soliped animals. 



A. Chakactbrs COMMON TO ALL THE EiBS. — These will be Studied frc-2n a 

 typical point of view, first in the rib itself, and then in its cartilage. 



1. Description of a typical rib. — A rib is an elongated symmetrical bone, 

 oblique from above to below, and from before to behind, flattened on both 

 sides, curved like a bow, and twisted on itself in such a fashion that its two 

 extremities cannot rest on the same horizontal plane. It is divided into a 

 middle portion and two extremities. 



Middle portion. — This oifers two faces and two borders. The external 

 face is convex, and hollowed by a wide groove in its anterior half; it shows 

 superiorly, towards the point corresponding to the angle of the rib in Man, 

 some tubercles and muscular imprints. The internal face is concave and 

 smooth, and covered by the pleura, which separates it from the lungs. The 

 anterior border is concave, thin, and sharp ; the posterior, convex, thick, and 

 covered with rugged eminences, is channeled inwardly by a vasculo-nervous 

 fissure, which disappears near the middle of the rib. 



Extremities. — The superior has two eminences, a head and a tuberosity, 

 which serve for the support of the rib against the spine. The first is formed 

 by two articular demi-faoets, placed one before the other, and separated by a 



