1026 



THORAX. 



the inferior border is more thin and sharp, 

 particularly in the middle third, or body, of the 

 bone. This thin or blade-like appearance is 

 caused by a groove on its inner aspect termed 

 sulcus costalis, which is commonly said to be 

 for the lodgment of the intercostal vessels : this 

 border also gives attachment to the intercostal 

 muscles. The borders are irregular in their 

 direction corresponding with the shape of the 

 rib ; which we shall presently notice. 



Fig. 663. 



Fourth rib. 



Extremities. (a) Posterior or vertebral 

 extremity. (Jiff. 663. c). This is rougher and 

 somewhat thicker than the other parts of 

 the rib, and is hence denominated its head 

 (capitulum costce). It presents, except in in- 

 stances to be presently stated two articular 

 facets, a superior and an inferior one, sepa- 

 rated by a welJ-defined ridge. Each of these 

 facets articulates with a corresponding small 

 surface on the bodies of two vertebrae, the 

 ridge just mentioned corresponding with the 

 intervertebral substance. 



The head of the rib is supported by a 

 narrow round part, somewhat constricted 

 the neck (fig. 663. /). This is flattened from 

 before backwards and is the weakest part of 

 the bone. Behind the neck there are some 

 inequalities, which correspond to the trans- 

 verse process of the dorsal vertebra below. 

 Externally to the neck is an eminence 

 known as the tubercle of the rib (tuberos- 

 ity, tuberculum costce, Jig. 663. g), which is 

 smooth in one part for its articulation with 

 the transverse process of the lower of the two 

 vertebras to which the head is connected, and 

 rough in the other, which is posterior, and in 

 some ribs superior to the above, for the inser- 

 tion of the posterior costo-transverse liga- 

 ment. The tubercles are most prominent in 

 the four or five superior ribs. Anterior to 

 the tubercle the rib suddenly bends forwards, 

 leaving this part the most convex, making 

 what is termed its angle, (fig. 663. K) The 

 interval which separates the tuberosity from 

 the angle, is the thickest, roundest, and 

 strongest part of the bone. 



(b) Anterior, or sternal extremity (fig. 663. 

 d). The anterior extremity of the ribs is 

 broad, flat, and deeply hollowed out at its 

 tip into an oval pit, into which is implanted 

 the costal cartilage. This extremity of the 



rib is broader and thicker than it is an inch 

 more posteriorly. 



Body. This may be described as that 

 part intervening between the angle and the 

 anterior extremity. We have stated that the 

 posterior end is more round than the anterior ; 

 the body, therefore, may be considered as of 

 a transition form, passing from the cylindrical 

 to the flat, blade-like, shape as it approaches 

 towards the sternum. 



Curve. The curve which the ribs follow 

 is very irregular, and therefore not easy to 

 describe. No doubt they are of the form 

 best adapted to admit of a great increase of 

 thoracic capacity at the expense of a remark- 

 ably small movement. They appear to encom- 

 pass the thorax in a somewhat spiral manner 

 (see dark lines figs. 682 and 683.) ; to accom- 

 plish which they have three curves, one the 

 common general arch or bend of the bone ; 

 the others the twist of the edges near the 

 extremities termed the curve of torsion. 



(A) Arch or general bend of the ribs. This 

 is the most remarkable feature of a rib. What- 

 ever be the curve of the 1st rib, it may 

 be said that each inferior rib describes a 

 curve " one size " larger ; so that one rib 

 can be laid close within the other, like hoops 

 of gradually increasing sizes. Two distinct 

 curves will then be seen. It will be ob- 

 served that the part extending between the 

 head and the angle describes a larger cir- 

 cle than the angle itself, which as the name im- 

 plies, is the most acute turn in the bone. 

 More anterior to this, the curve becomes 

 remarkably large ; which Haller has so ex- 

 pressively described as representing the tan- 

 gent to the posterior curve. In connexion 

 with the general curve of the rib should be 

 noticed two linear measurements, viz., the 

 chord and the versed sine. In fig. 664. is given 



Fig. 664. 



Second rib. 



the 2nd rib ; the line A B is the chord, and 

 D c the versed sine, or a line extending from 

 the chord to the most prominent part of the 

 bend of the bone. The general curve regulates 

 the, length of the versed sine. 



This curve of the rib gives the sides of the 

 chest a power of enlarging, a lateral mobility, 

 according to the length of the versed sine, 

 and quite distinct from the antero-posterior 

 enlargement, which is according to the length 

 of the chord. The ribs do not increase the 

 lateral dimensions of the thorax by abduction, 

 but solely by their rotation upon the line A B 



(B) Curves of torsion of the ribs. If we take 



