THE THORAX. 9S 



The first is formed by two articular demi-facets, placed one before the other, and 

 separated by a groove for ligamentous insertion ; it is isolated from the tuberosity 

 by a narrow part, named the neck, which exhibits a rugged fossa for the implantation 

 of a ligament. The second — situated behind the head, and smaller than it — is 

 provided with imprints on its margin, and presents an almost flat diarthrodial facet 

 at the summit. Each rib articulates by its head and tuberosity with two dorsal 

 vertebrse ; the head is received into the intervertebral articular cavity ; the tube- 

 rosity corresponds, by its facet, to the transverse process of the posterior vertebra. 



The inferior extremity is tuberous and excavated by a shallow cavity, 

 irregular at the bottom, for the reception of the upper end of the costal cartilage. 



Structure and development. — The ribs are very spongy bones, especially in their 

 inferior moiety, and are developed at a very early period from three centres of 

 ossification — a principal for the middle portion and inferior extremity, and two 

 complementary for the head and tuberosity. 



2. Description of a typiccd costal cartilage. — The costal cartilage QicBmapopliysis) 

 very evidently represents the inferior rib in Birds ; it is a cylindrical piece, 

 slightly compressed at the sides, and round and smooth on its faces and borders. 

 By its superior extremity, it is united to the rib it serves to lengthen, and forms 

 with it an angle more or less obtuse, opening in front. At its inferior extremity, 

 it is terminated by an articular enlargement, or by a blunt point. In youth, the 

 costal prolongations are entirely composed of cartilaginous matter, but they are 

 soon invaded by ossification ; so that in the adult animal they are already trans- 

 formed into a spongy, substance, with large areolae which remain during life 

 surrounded by a thin layer of cartilage. 



B. Specific Characters of the Ribs. — The ribs, hke the vertebra of 

 each region of the spine, have received numerical designations of first, second, 

 third, etc., computing them from before to behind (Fig. 5). Owing to the 

 presence of an altogether essential characteristic, they are naturally divided into 

 two great categories — the sternal or true ribs, and the asternal or false ribs. The 

 sternal ribs, numbering eight — the first eight — have their cartilages terminated 

 inferiorly by an articular enlargement, which corresponds to one of the lateral 

 cavities of the sternimi, and brings the true ribs into direct contact with this 

 portion of the skeleton. The asternal ribs, ten in number, rest on each other — 

 the last on the seventeenth, that on the sixteenth, and so on — by the inferior 

 extremity of their cartilage, which ends in a blunt point. The cartilage of the 

 first false rib is united somewhat closely to the last sternal rib, and it is through 

 the medium of this that all the asternal ribs lie indirectly on the sternum. 



If, however, the ribs are considered altogether, with regard to the differential 

 characters presented by them in their length, width, and degree of incurvation, 

 it will be noted : 1. That their length increases from the first to the ninth, and 

 from this diminishes progressively to the last, 2. That the same progressive 

 increase and decrease exists in the cartilages. .3. That they become gradually 

 wider from the first to the sixth inclusive, and then contract by degrees until the 

 eighteenth is reached : 4. That the curve described by each is shorter and more 

 marked as the rib is situated more behind. It may be added, that the channel 

 on the external face is less conspicuous in proportion as the rib is narrow. 



The first rib, considered individually, is always distinguished by the absence 

 of the groove on its outer surface, by the vasculo-nervous groove on its posterior 

 border, and the groove or notch intermediate to the two facets of its articular 

 head. It is also recognized by the deep muscular imprints on its external face, 



