26 BONES OF THE THORAX. 



front, and is most so in the neighbourhood of the angle. At first the rib is 

 directed backwards as well as outwards. Besides the main curves now 

 mentioned the rib is somewhat twisted on itself, so that the two extreme 

 portions cannot be made to rest at the same time on a flat surface. It 

 follows, from the same twisting in the majority of the ribs, that while their 

 surfaces are vertical posteriorly, they are sloped upwards and inwards at 

 the anterior part. 



CHARACTERS PECULIAR TO CERTAIN BIBS. The ribs increase in length from 

 the first to the eighth, and decrease from the ninth to the twelfth, so that the 

 last is little longer, often even shorter, than the first. The first rib is the 

 broadest, and after it the middle ones ; the twelfth is the narrowest. The 

 distance of the angle from the tubercle increases gradually from the second 

 to the eleventh ; in the first the angle coincides with the tubercle, in the 

 last it is not perceptible. 



The first rib is shorter and broader than any of the others. Its direction 

 is slightly inclined, and its surfaces lie nearly in the same flat plane. Its 

 superficial and thoracic surfaces look respectively upwards and downwards. 

 The head is small, and presents a single articular surface. The neck is 

 round and slender. It articulates with the body and transverse process of 

 the first dorsal vertebra alone. On the snpeiior or superficial surface are two 

 very slight smooth depressions with an intervening rough mark, and a 

 considerable rough surface behind. The rough surface marks the attachment 

 of the scalenus medius muscle, the posterior depression the position of the 

 subclavian artery, the anterior depression the subclavian vein ; and the inter- 

 vening slight elevation, frequently terminating in a sharp spine on the inner 

 edge, indicates the attachment of the scalenous anticus muscle. 



The second rib is longer than the first, and presents externally a prominent 

 roughness which marks the attachment of the senatus magnus. 



The eleventh and twelfth are distinguished as the free or floating ribs, 

 because their cartilages are pointed and unattached in front. They have no 

 groove on the inferior border, no articular tubercle, and only one articulating 

 surface on the head. The mark of the angle is scarcely perceptible on the 

 eleventh, and is absent from the twelfth. 



Varieties. The number of the ribs is sometimes increased to thirteen on one or 

 both sides. The supernumerary rib is most frequently very short, and is formed in 

 connection with the transverse process of the first lumbar vertebra, or occasionally 

 with the seventh cervical : in the latter case the additional rib has a double attach- 

 ment, viz., to the body and transverse process of the vertebra. The number is 

 sometimes diminished to eleven pairs by the absence of the twelfth. Ribs likewise 

 occur greatly expanded or forked at their anterior extremities ; and occasionally two 

 or more ribs are united together in a part of their extent by their margins. 



The COSTAL CARTILAGES are continuations of the rtt>s. Their breadth 

 diminishes gradually from the first to the last, whilst the length increases 

 as far as the seventh, after which it becomes gradually less. Their line of 

 direction varies considerably. The first descends a little, the second is 

 horizontal, and all the rest, except the last two, ascend more and more from 

 the rib towards the sternum as they are situated lower down. The external 

 or costal extremity, convex and uneven, is implanted into and united with 

 the end of the corresponding rib. The internal extremities of the upper 

 seven (except the first), are smaller than the external and somewhat pointed, 

 and fit into the corresponding angular surfaces on the side of the ster- 

 num, with which they are articulated in synovial cavities. Each of the 

 cartilages of the first three asternal ribs becomes slender towards its ex- 



