112 



OSTEOLOGY. 



but faintly marked. The anterior extremity of the rib is thickened and often ex- 

 panded for the reception of its costal cartilage, which is not infrequently ossified. 

 Here, on the upper surface, are attached the cos to-clavicular ligament and the sub- 

 clavius muscle. The inferior surface of the rib is smooth and is covered with 

 pleura. The outer convex border, thin in front, is usually thick and rough behind 

 the subclavian groove, where it has attached to it the fibres of the first digitation of 

 the serratus anterior. Along this edge, also, are attached the external and internal 

 intercostal muscles of the first intercostal space. The inner concave border is 

 thin, and has connected with it the aponeurotic expansion known as Sibson's fascia. 

 The second rib may be distinguished by the sharpness of its curve ; the absence 

 of any twist on its body, so that it can be laid flat on the table ; the oblique direction 

 of the surfaces of its body, the outer being directed upwards and outwards, whilst 

 the inner is turned downwards and inwards ; and the presence of a well-marked, 

 rough, oval area about the middle of its external surface and lower border for part of 

 the first, and the whole of the second digitation of the serratus anterior muscle. 

 The head has two facets, and the angle is close to the tubercle posteriorly. 



The tenth rib has usually only a single articular facet on the head, and may or 

 may not have a facet on the tubercle. 



The eleventh and twelfth ribs are recognised by their length. The head of 

 each is usually large in proportion to the body; it supports a single facet for 

 articulation with the eleventh or twelfth thoracic vertebrae. The tubercles are ill- 

 developed and have no articular facets. The angle is faintly marked on the 



eleventh, scarcely per- 

 ceptible on the twelfth. 

 The anterior extremities 

 of both are narrow and 

 pointed and tipped with 

 cartilage. The costal 

 groove is absent in the 

 twelfth, and but slightly 

 seen in the eleventh. 

 The twelfth is consider- 

 ably shorter than the 

 eleventh rib. 



Ossification. Os- 

 sification begins in the 

 cartilaginous ribs about 

 the sixth week, and rapidly 

 extends along the body, so 

 that by the end of the 

 third month it has reached 

 the permanent costal car- 

 tilage. The sixth and 

 seventh ribs are the earli- 

 est to ossify ; the first 

 rib being the last (Lam- 

 bertz). At puberty, or 

 before, secondary centres 

 appear. One for the head. 

 In the first rib there is one 

 epiphysis for the tubercle. 

 In the second to the sixth 

 ribs inclusive there are 

 two epiphyses for the 

 tubercle, one for the ar- 

 ticular part and one for 

 the non-articular part. In the remaining ribs which have articular tubercles there is only 

 one epiphysis (Fawcett). By the twenty-fifth year fusion between the epiphyses and the 

 body is complete. 



FIG. 126. THE THORAX AS SEEN FROM THE FRONT. 



