52 THE CAT. [CHAP. in. 



The a))r/ks of tlic ribs become more and more distant from the 

 tubercula as we pass backwards to the eleventh rib. The thirteenth 

 exhibits no angle. 



§ 17. The COSTAL CAUTILAGES (Fig. 24, ca), differ much as to 

 length, connexion, shape and direction. The tenth is tlie longest, 

 and tlience the length decreases as we pass either forwards or 

 backwards through tlio scries. The first nine join the sternum. 

 That of the tenth rib joins the costal cartilage of the ninth rib, 

 and similarly tlie eleventh and twelfth costal cartilages unite distally 

 with the lower border of the costal cartilage next in front. The 

 thirteenth costal cartilage ends freely. The first costal cartilage 

 is the broadest, and thence they gradually narrow backwards. The 

 last cartilage is pointed at its distal end. The upper (proximal) 

 end of each costal cartilage is convex, and fits into the distal con- 

 cavity of its rib. As to direction, the cartilages pass at first 

 backwards, then downwards, curving distally forwards from the 

 fourth to the seventh. The first cartilage has a nearly horizontally 

 forward direction, while the last extends downwards and backwards. 



§ 18. The THORAX AS A WHOLE fomis a long, transversely narrow, 

 conical case, with a small aperture in front and a wide oblique 

 opening behind. It is considerably deeper from above downwards 

 than it is wide from side to side. The variation in its dimension, 

 which shows itself as we proceed, from before backwards through 

 the thorax, is produced by the corresponding variation in the length 

 of the ribs and in their curvature. The anterior opening is bounded 

 by the first pair of ribs, the first dorsal vertebra and the manubrium. 

 The posterior opening is bounded by the xiphoid process, the cartilages 

 of the four hindmost ribs, the body of the thirteenth rib and the 

 thirteenth dorsal vertebra. 



§ 19. {Such being the structure of the bony and cartilaginous parts 

 which make up the spinal portion of the axial skeleton, we have 

 next to consider the fibrous bands, or ligaiments, which hold together 

 the bones and cartilages already described. The substance inter- 

 posed between each pair of true vertebra is an elastic body termed 

 an i)ifcrfcrtcbnil (//•sc. 



Each such disc is made up of concentric lamella) (Fig. 26, ./'), of 

 fibro-cartilaginous and fibrous tissue, surrounding a soft central por- 

 tion if/), which is very elastic (projecting beyond the general level of 

 the disc when pressure is removed) and contains immerous nu- 

 cleated corpuscles like those of cartilage. 



The surface of each centrum is covered (except towards its cir- 

 cumference) with a thin layer of cartilage, and it is to it that the 

 intervertebral discs are attached. 



These discs form so many elastic pads, and one such is placed 

 between each pair of presacral vertebra), except between the atlas 

 and the axis. 



A strong band of fibres, called the roitral coiumon /if/nijioif, extends 

 along the ventral surface of the vertebral bodies. It is thickest 

 where it passes over the middle of the centra than elsewhere, and 



