THE THORAX. 69 



in their inferior moiety, and are developed at a very early period by 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 typical costal cartilage.— The costal cartilage 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 enlarge- 

 ment, 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 transformed into a spongy 

 substance, with large areolre which remain during life surrounded by a thin 

 layer of cartilage. 



B. Specific Characters of the Eibs. — The ribs, like the vertebrae of each 

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

 etc., computing them from before to behind. (See Pig. 1.) Owing to the pre- 

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

 two great categories: the sternal or true ribs, and the atternal or false ribs. 

 The sternal ribs, numbering eight (the first eight), have their cartilages 

 terminating inferiorly by an articular enlargement, which corresponds to one 

 of the lateral cavities of the sternum, 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, this 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 difieren- 

 tial 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 con- 

 tract 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, the vasculo-nervous fissure on its 

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

 its articular head. It is also recognised by the deep muscular imprints on 

 its external face, the shortness and thickness of its cartilage, and particularly 

 by the articular facet which this cartilage exhibits inwardly, to cpiyespond 

 to that of the opposite rib. The last rib has no channel on its 'external 

 surface ; the facet of its tuberosity is confounded with the posterior facet (jf 

 the head. This last character is- also nearly always remarked in the 

 Seventeenth rib, and sometimes even in the sixteenth. 



In the Ass and Mule, all the ribs in general, but particularly those most 

 posterior, are less curved than in the Horse. (In the Horse, a nineteenth 

 pair of ribs is sometimes found, and this even with five, and at other times 

 with six lumbar vertebrse ; it happens that the nineteenth rib is formed by 

 the transverse process of the first lumbar vertebra, and at times a ligament 



