THE THORAX. 93 



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

 than behind, offers to the powerful pectoral muscles a large surface for insertion.^ 



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

 faces ; they are situated above the diai-throdial cavities, are united anteriorly, and 

 each gives attachment to a fibrous band. 



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 con- 

 stitutes the cervical prolongation {prcesternum) of the sternum. The posterior 

 extremity is flattened superiorly and inferiorly, and forms a large cartilaginous 

 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 



Fig. 56. 



THE STERNUM. 



1, The cervical prolongation (prcesternum, or cariniform cartilage) ; 2, the xiphoid appendage (or 

 ensiform cartilage); 3, 3, cavities for the articulation of the sternal cartilages ; 4, inferior burder. 



which do not undergo complete osseous transformation. It is developed, in 

 Solipeds, from six single nuclei of spongy substance, ranged one behind the 

 other, like beads on a string. These nuclei never coalesce to form a solid piece, 

 but remain separated during the life of the animal, by the primary cartilaginous 

 mass. The latter constitutes the entire anterior prolongation of the bone and its 

 carina, as well as the xiphoid appendage. When these parts of the sternum 

 become ossified — which is rare — they are only partially so. 



Differential Characters in the Sternum op other Animals. 



In all the domesticated animals except Solipeds, tlie sternum is flattened above and below 

 instead of ou both sides. 



A. Ruminants. — In Ruminants, each piece is developed from two lateral centres of ossifi- 

 cation. The bones wliich compose it are seven in number; they are much more compact than 

 those in the sternum of the horse, and at an early period are united to each other, with the 

 exception of tl:e first, which is joined to the secmid by a diarthrodial articulation tliat permits 

 it to execute lateral movements. There is no cervical prolongation, and the xiphoid cartilage 

 is feebly developed and well detached from the bodv of tiie bone. In the sternum of the Goat 

 and Sheep, the two first pieces have no diarthrodial joint, but are simply united by a layer of 

 cartilage, which, in old animals, becomes completely ossified. The sternum of the Camel has 

 a very oblique direction downwards and backwards, and is formed by seven pieces ; the first is 

 somewhat rudimentary, while the two are very strong, and serve as a base for the sternal 

 callosity or pad. 



B. Pig. — Tlie sternum of this animal presents in its general conformation the essential 

 features of that of large Ruminants. It is provided with a well-defined cervical prolongation, 

 and is composed of six pieces, which, at least in the four or five last, are each divided into two 

 lateral nuclei. 



C. Carnivora. — ^The sternum of the Dog and Cat is formed of eight pieces, elongated from 

 before to beliiud, hollowed in their middle part, and thick at their ends— formed, indeed, like 

 the last coccygeal vertebrae of the Horse. They are never ossified to each other. 



9 



