THORAX AS A WHOLE. DEVELOPMENT. 



27 



tremity, and is attached to the lower border of that which is next above it. 

 The last two are pointed and unattached. 



The first cartilage, which is directly united to the sternum without 

 articular cavity, usually becomes more or less ossified in the adult male ; and 

 the others likewise exhibit a considerable tendency to ossify in advanced 

 life. This tendency is not so great in the female, in whom costal respiration 

 is generally more extended than in the male. 



THE THORAX AS A WHOLE. 



The thorax is of a somewhat conical shape, with convex walls. Its upper 

 inlet is contracted, and bounded by the first dorsal vertebra, the first pair of 

 ribs, and the manubrium of the sternum. Its inferior margin curves down- 

 wards and backwards on each side from the ensiform process to the twelfth 



Fig. 23. 



Fig. 23. FRONT VIEW OF THE THORAX, 



SHOWING THE STERNUM COSTAL CARTI- 

 LAGES, RIBS, AND DORSAL VERTEBRA, i 

 1, the manubrium, or first piece of the 



sternum ; 2, is close to the place of union 



of the first costal cartilage ; 3, the clavicular 



notch ; 4, the middle of the body of the 



sternum ; 5, the ensiform piece ; 6, the 



groove on the lower border of the ribs ; 7, 



the vertebral end of the ribs ; 8, the neck ; 



9, tubercle; 10, costal cartilage; 12, the 



first rib; 13, its tuberosity; 14, the first 



dorsal vertebra ; 15, the eleventh ; 16, the 



twelfth rib. 



rib. Its longitudinal axis is directed 

 upwards and slightly backwards. Its 

 transverse diameter, at its widest part, 

 greatly exceeds that from before back- 

 wards. The latter is shortened in 

 the middle line by the projection of 

 the vertebral column, but on each 

 side of the column a considerable ex- 

 tension of the cavity is produced by the backward direction of the posterior 

 parts of the ribs, and thus the weight of the body is thrown further back 

 and more equally distributed round the vertebral column. At the same 

 time a broad furrow is produced between the spines of the vertebrae and the 

 angles of the ribs, in which are placed the erectores spinse muscles. 



DEVELOPMENT OF THE RIBS AND STERNUM. 



The first origin of the posterior parts of the ribs in connection with the 

 primordial vertebrae has been already mentioned. In the second month 

 cartilage appears in them, deposited independently of the vertebrae. 

 Different opinions have been held as to whether the tubercle or the head 

 of the rib is the primary point of its connection with the vertebra : it appears 

 probable that the whole extent from head to tubercle is an expansion of the 

 original connection. (Kolliker, "Entwicklungsgeschichte," fig. 81 ; Rathke, 

 "Entwick. der Natter;" Huxley, "On the Theory of the Vertebrate Skull," 

 p. 74.) After becoming cartilaginous, the ribs pass round in the visceral 

 wall, and, according to Rathke, before reaching the front, the first seven on 



