THE COSTAL CARTILAGES 47 
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 common 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 Tubercle 
and rounded. The groove of the lateral surface is He 
a. : 6 4 ead 
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- ™ Neck 
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 Sternal 
posterior ribs. Sr ea 
Anterior 
border 
Cartilage 
Variations.—A nineteenth rib on one side or both is not 
atallrare. It is usually imperfectly developed and quite vari-  pyg. 24 Terr First Rip or Horse; 
able. In many cases it is a mere strip of cartilage connected ; Ei ; 
by ligament with the first lumbar transverse process; in 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 thoracic or lumbar or ambiguous 
in character. It is commonly floating, but may be attached to the eighteenth costal cartilage. 
Reduction in number isuncommon. In rare cases the first rib is imperfectly developed and does 
not reach to the sternum. Partial fusion of adjacent ribs and other anomalies occur. 
Meprat VIew. 
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 
