THE THORAX AS A WHOLE. 29 



is implanted into and united with the end of the corresponding rib. The internal 

 extremities of the cartilages of the true ribs (except the first, which is directly united 

 to the sternum without articular cavity) are smaller than the external, and fit into the 

 corresponding notches on the side of the sternum. Each of the cartilages of the first 

 three false ribs becomes slender towards its extremity, and is attached to the lower 

 border of that which is next above it. The eleventh and twelfth are pointed and 

 unattached. The fifth, sixth, seventh, and eighth cartilages form a series of inter- 

 chondral articulations, by means of a broad process sent down from the rounded angle 

 of the one meeting a less salient projection from the upper border of the next. 



Varieties. The eighth costal cartilage not unfrequently articulates with the sternum. 

 Occasionally the seventh costal cartilage fails to reach the mesosternum (owing to reduction 

 of the latter), and meets its fellow in front of the ensiform process. The articulation 

 between the fifth and sixth cartilages is sometimes wanting, and then one may be formed 

 between the eighth and ninth. 



THE THORAX AS A WHOLE. 



The bony thorax is of a somewhat conical shape, flattened from before back, and 

 much longer behind than in front. The posterior wall, formed by the dorsal 



Fig. 29. FRONT VIEW OF THE THORAX. 



1, manubrium ; 2, is close to the place of union of the 

 first costal cartilage ; 3, clavicular notch ; 4, body of the 

 sternum ; 5, ensiform process ; 6, groove on the lower 

 border of the ribs ; 7, the vertebral end of the ribs ; 8, neck ; 

 9, tuberosity; 10, costal cartilage; 12, first rib; 13, its 

 tuberosity ; 14, first dorsal vertebra ; 15, eleventh rib ; 16, 

 twelfth rib. 



vertebrae and the ribs, is convex from above 



down, and, the ribs being directed backwards 



from the vertebrae as far as their angles, a broad 



furrow is produced on each side of the spines, which 



lodges the erector spinae muscle. The anterior 



wall, formed by the sternum and costal cartilages, 



is only slightly convex, and is inclined at an 



angle of 20 to 25 with the vertical. In the 



condition of expiration the upper border of the 



sternum is opposite the disc between the second and third dorsal vertebrae, the 



junction of the manubrium and body opposite the middle of the fifth dorsal vertebra, 



and the xiphi-sternal articulation about opposite the interspace between the ninth 



and tenth dorsal vertebra?. 



The sides are sloped outwards to about the ninth rib, are slightly convex from 

 above down, and strongly arched from before back. The upper aperture is 

 contracted, reniform, nearly plane, and much sloped downwards ; the lower is 

 irregular ; the margin ascends on each side from the tenth rib to the xiphi-sternal 

 articulation, and thus gives rise to the subcostal angle, in the centre of which the 

 ensiform process projects. The form of the cavity corresponds generally to that of 

 the exterior, but in the median plane the antero-posterior diameter is much reduced 

 by the projection of the bodies of the vertebras ; as a consequence of this and the 

 backward direction of the hinder ends of the ribs, a deep hollow is formed on each 

 side, into .which the posterior portions of the lungs are received, and thus the weight 

 of the body is thrown farther back and is more equally distributed around the 

 vertebral column. 



