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

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

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

 than behind, offers to the powerful pectoral muscles a largo surface for 

 ion. 



H'H-iIers. The two lateral borders separate the superior from the lateral 



: 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. 



E.rfi-c/nitii's. 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 post> riof 

 iiity is flattened superiorly and iuferiorly, and forms a large cartila- 

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

 name of the abdominal prolongation (juuiform cartilage) or xiplioid appendage. 



- 'iiri'uml '!< / lapment. 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. These centres never coalesce to form a 

 s.did 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 cariiia, as well as the xiphoid appendage. When 

 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 eighteen ribs are 



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



intervals termed the iufi-ri-nxlal spaces. Attached by their superior extremity 



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



niity by an elastic and flexible prolongation, named the costal cartil<t<j<\ 



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



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



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



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



A. CHARACTERS COMMON TO ALL THE KFBS. These will be studied from a 

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



1. xMMrtptioM i 'fa li/jiii-nl rib. A rib is an elongated symmetrical bone, 



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



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



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



ntiilill-- /XT/ i<iu and two extremities. 



M5adle portion. This offers two faces and two borders. The external 



nvex, and hollowed by a wide groove in its anterior half; it shows 



fi|i. -riorly. towards the point corresponding to the angle i.f the rib in Man. 



- and muscular imprints. The internal face is concave and 



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



r is concave, thin, and sharp : the posterior, convex, thi'-k. and 



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



. nhich disappears near the middle of the rib. 



x. The superior has two eminences, a Juwl and a inl< 



which M-rvo for the snpportof the rib again.-t the spine. The fust i> !' m:ed 

 by two articular demi-fa-ets. placed <>n, I. .-fore the other, and separated by a 



' i- 'I 



