THE COSTAL CARTILAGES 



47 



Tubercle 

 Head 



Neck 



extensive articular surface which is convex in its length. The sternal end is larger 

 than that of any other rib; it is thick and very wide, and is turned a little forward. 



The last rib is the most slender and regularly curved. It is usually but little 

 longer than the second. The facet on the tubercle is confluent with that of the head. 

 (This feature, however, is comrnon on the seventeenth also, and may occur on the 

 sixteenth.) 



The serial position of the other ribs may be determined approximately by the fol- 

 lowing considerations : The length increases from the first to the tenth and eleventh 

 and then diminishes. The width increases somewhat to the sixth and then diminishes. 

 The anterior border is thin and sharp from the 

 second to the eighth, and behind this becomes thick 

 and rounded. The groove of the lateral surface is 

 distinct on the fourth to the eighth inclusive. The 

 curvature increases in degree rapidly from the second 

 to the seventh, remains about the same to the six- 

 teenth, and then decreases very noticeably. In re- 

 gard to dorso-ventral direction, the first rib inclines 

 a little forward, the second is about vertical, while 

 behind this they slope backward in increasing de- 

 gree, so that a transverse plane tangent to the ventral 

 ends of the last pair cuts the third lumbar vertebra. 

 The head and tubercle diminish in size from first to 

 last. Their relative positions change, in that the 

 tubercle of the first rib is almost directly lateral to 

 the head, while further back it gradually comes to 

 lie behind it. The neck is longest on the longest 

 ribs, and is absent on the last two or three. A 

 costo-transverse foramen (Foramen costo-transver- 

 sarium) is formed between the neck and the trans- 

 verse process. 



Development. — The ribs ossify in cartilage from 

 three centers, one each for the shaft, head, and 

 tubercle; the third center is absent in some of the 

 posterior ribs. 



Anterior 

 border 



Sternal 

 extremity 



Variations. — A nineteenth rib on one side or both is not 

 at all rare. It is usually imperfectly developed and quite van- 

 able. In many cases it is a mere strip of cartilage connected 

 by Ugament with the first lumbar transverse process; m other 

 cases it is well developed, and may be fused with the process; 

 in others again it is connected with a vertebra which may be 



Cartilage 



Fig. 24. 



-Left First Rib of Horse; 

 Medial View. 



thoracic or lumbar or ambiguous 



in character. It is commonly floating, but may be attached to the eighteenth costal cartilage 

 Reduction in number is uncommon. In rare cases the first rib is imperfectly developed and does 

 not reach to the sternum. Partial fusion of adjacent ribs and other anomahes occur. 



THE COSTAL CARTILAGES 



The first costal cartilage is an inch or more (2.5 to 3 cm.) in length. The 

 dorsal end is very wide and thick. The sternal end is small. The two articulate 

 with each other as well as with the sternum. The cartilages of the other sternal 

 ribs increase progressively in length and become more rounded. The sternal end 

 is enlarged and has an elliptical convex facet for articulation with the sternum. 

 The cartilages of the asternal ribs are long, slender, and pointed. The ninth is 

 very firmly attached to the eighth; it and the next two are the longest and behind 

 this they diminish progressively in size. The cartilages of the asternal ribs overlap 

 and are attached to each other by elastic tissue, forming the costal arch (Arcus 



