46 



A MANUAL OF ANATOMY 



Ihe ribs are supplied with blood by branches of the intercostal 

 arteries. 



Structure. — A rib is composed of loose cancellated tissue sur- 

 rounded by compact bone. 



Varieties. — (i) The number may be increased to thirteen on one or both sides, 

 and the supernumerary rib may be cervical or lumbar. If cervical, it is 

 d e veloped in connection with the costal process of the seventh cervical vertebra. 

 It may join the shaft of th3 first thoracic rib, or it may reach the sternum. If 

 lumbar, it is developed in connection with the costal element of the first lumbar 

 vertebra, is usually very short, and does not articulate with the body of that 

 . , . . . ^ . ,r vertebra. (2) In rare cases the 



Appears about Appear about the i6th Year, , il j j t_ 



the 8th Week .^,-., and join about the number may be decreased by one, 



(intra-uterine)^ r/^ip, \ 25th Year at the expense of the twehth rib. 



(3) The ribs are subject to variety 

 in form as follows : (a) the verte- 

 bral end of the first thoracic rib 

 may be joined by a cervical rib, or 

 by the vertebral end of the second 

 rib, in which cases the variety 

 known as bicipital rib occurs; 

 {b) the anterior extremity of a rib 

 may be bifurcated; (c) adjacent 

 ribs may be connected by small 

 plates of bone. 



Ossification. — ^An ordinary rib 

 has one primary centre and three 

 secondary centres. The primary 

 centre for the shaft appears 

 about the sixth week near the 

 angle. Ossification proceeds so 

 rapidly along the shaft that by 

 the fourth month the shaft is 

 completely ossified. The second- 

 ary centres appear about the six- 

 teenth year. One gives rise to the head, and of the other two one is for the 

 rough part of the tubercle and the other for its articular part. The head and 

 two parts of the tubercle join the shaft about the twenty-fifth year. The two 

 secondary tubercular centres are absent in the eleventh and twelfth ribs. 



The Costal Cartilages. 



The costal cartilages, which are composed of hyaline cartilage, 

 are twelve in number on either side. The outer extremity of each 

 is received into the oval pit on the anterior extremity of a rib, 

 and is there maintained in position by the continuity which takes 

 place between the periosteum of the rib and the perichondrium of 

 the cartilage. The inner extremities of the true ribs articulate 

 with the side of the sternum by means of synovial joints, except 

 in the case of the first, which is directly united to the presternum 

 without the intervention of a synovial membrane. The eighth 

 as a rule, ninth, and tenth do not reach the sternum, and they 

 articulate with each other by synovial joints, each cartilage being 

 widened at the place of articulation, where it sends downwards a 

 • process to the upper border of the cartilage below. In this way 

 interchondral joints are formed between these cartilages, as well 



Fig. 32. — Ossification of a Rib. 



