THE STERNUM. 



163 



number. The costicartilages wliich reach the sternum articulate therewith diarthrodially, 

 but there is considerable diversity as to the extent of the synovial capsules. 



Each of the constant mescsternebrae is two or three times as long 

 as wide, and slightly enlarged at the ends so as to be somewhat of a pfS^SK/TICM 



dumb-bell shape. 



§ 425. PrcESternum. — This is sometimes called manubrium from 

 its form in man. It is nearly twice as long as the average mesoster- 

 neber, and the cephalic half, which is cartilage in the kitten, is com- 

 pressed and tapers to a blunt point. On each side is an oblique 

 shoulder for the attachment of the first costicartilage, so that the 

 entire prsesternum is shaped somewhat like the head of a lance. Its 

 ventral aspect is prominent on the meson, forming the presternal keel. 



§ 426. XipMsternum. — This is also called the xiphnd or endform 

 cartilage. In the adult cat only the caudal third or fourth is cartilage, 

 and its tip is enlarged into a disk. The rest of the siphisternum 

 tapers caudad from its base. 



The sternum affords attachment to the MM. ectopectoralis, entopec- 

 toralis, sterno-mastoideu.% sterno-Jiyoideus and some others. 



§ 427. Relation of the Sternebrce to the Costicartilagines. — While 

 dissecting, it is often desirable to designate the nimiber of a sterneber 

 when the sternum is so covered by muscles as to make the enumera- 

 tion difficult. In these cases the ribs or their cartilages may usually 

 be counted from the most cephalic of the series, their relations to the 

 sternebrae being as follows :— 



With the six constant mesosternebrse, the cartilages are attached 

 at the cephalic end, so that the third mesosterneber, for example, 

 which is the fourth sterneber, would be the segment just caudad of 

 the point of attachment of the fourth cartilage. The first cartilages 

 are connected with the sides of the prsesternum, which is really the 

 first sterneber ; the relations of the eighth and ninth cartilages are 

 less definite and constant. 



COST^ (RIBS) AND COSTICARTILAGINES (COSTAL CARTI- 

 LAGES). Fig. 30. 



References. — Straus-Durckheim, A, I, 492, and II, 57 ; Quain, 



A, I, 27, 141 ; Gray, A, 210, 295 ; Flower, A, 85 ; Chauvean (Fleming), 



A, 67, 140 ; Chauveau, A ; Leyh, A, 162, 209 ; Humphrey, A, 329-337 ; Wp/U^gfTlOL 



Mivart, B, 50-52. ^ .„ n, ,, 



Fig. 49. — i he V en- 



r. .^^ mi . .-1 ^ -, -, . 1 TRAL Aspect OF 



§ 428. The cost© or nos — 13, rarely 14, on eacn ax Adult Ster- 

 side — constitute a series of arched, highly elastic ntjm ; x i. 

 bones which, with their continuations, the costicar- 

 tilagines, the sternum and the thoracic vertebrae, form the conical 

 skeleton or framework of the thorax (Fig. 30 and 50). 



The ribs and their cartilages are slender and subcylindrical in 

 form. They present, in expiration and moderate inspiration, a com- 

 pound curve, the convexities being caudal and lateral (Fig. 30 and 



