STERNUM, OR BREAST-BONE 375 



increase in length and breadth to the ninth ; from the ninth to the last 

 they become more curved and short. In the seventeenth and eighteenth 

 the articulatory surface of the tubercle becomes confounded with that of 

 the head, and the neck is wanting (see .,^ ., B, Fig. 21). 



THE STERNUM, OR BREAST BONE 



The sternum, which forms the inferior boundary of the thorax (see page 

 403), forms in the adult one long keel-like bone, of a spongy or cellular 

 nature, more or less divided by cartilage ; but in the embryo it, like the 

 sacrum, is made up of distinct pieces, six or seven in number. Anteriorly 



Fig. 22.— Profile View of Sternum (Breast-bone). 



1. 1. 1. etc. Pits for attachment of costal cartilages. 3. 3. Posterior extremity witli its ensiform car- 



2. 2. Anterior extremity ending in cariniform tilage. 



cartilage. 



the breast-bone is prolonged in a manner resembling the keel and figure-head 

 of a ship, being clothed with cartilage, which is hence called cariniform 

 (carina, a keel) (o, 2, P'ig- 22). Posteriorly it terminates also in a cartilage 

 of a thinner and more flexible form, and called, from its resemblance to a 

 sword, ensiform (ensis, Lat. sword) or xiphoid {^i^os, Gk. sword). 



The sides are occupied above by ,the cavities for the insertion of the 

 costal cartilages, and below by a rough surface, to which the fibres of the 

 pectoi"al muscles are attached. The superior face forms a very lengthened 

 isosceles triangle, having its apex forward. It is slightly hollowed from 

 before backwards. 



The three anterior divisions of the sternum present broad lateral 

 surfaces ; the three posterior have projecting sides inclining downwards, and 

 forming the boundaries between the inferior surface and the sides. The 

 ribs are received into cavities in the cartilage existing between the separate 

 bones, the last posteriorly having, however, an additional pit in its middle 

 for the cartilage of the eighth rib. 



SCAPULA, BLADE-BONE OR SHOULDER-BLADE 



Intervening between the thorax and the fore extremity, and presenting 

 large surfaces for the attachment of muscles to connect these two parts of 

 the skeleton, is the scapula or shoulder-blade. It is a triangular flat bone, 

 and lies obliquely on each side of the anterior part of the thorax, with ita 



