BACKBONE 425 



of the succeeding vertebrae are fused with the true sacral vertebra, 

 the whole mass being known as the sacrum. The caudal region 

 contains about eighteen vertebrae, of which the first three or four 

 are fused with the sacral. They grow smaller from before back- 

 wards, losing their processes and becoming degenerate. 



RIBS AND BREASTBONE 



The ribs are present as independent elements only in the 

 thoracic region. They are curved, bony rods, articulated with the 

 vertebrae. Those of the first nine pairs are connected at their 

 lower ends with the breastbone by bars of calcified cartilage 

 known as their sternal portions or as sternal ribs. The end w^hich 

 articulates with the vertebra has a knob known as the head or 

 capitulum. The first nine pairs have a second facet on the dorsal 

 side at a short distance beyond the head. This is for articulation 

 with the transverse process of the vertebra ; immediately beyond 

 it, for the attachment of ligaments, is a short projection, together 

 with which it forms the tuberculum. The sternal portions of 

 the first seven pairs articulate directly with the sternum ; those 

 of the eighth and ninth are connected with the ribs in front of 

 them. The last three pairs have no sternal portions and no tuber- 

 cula. The breastbone or sternum is a long, narrow rod, divided 

 into segments, sometimes called sternebrae, and lying in the mid- 

 ventral line of the thorax. The first segment is the manubrium. 

 It is the largest and is flattened from side to side. Behind it 

 come four segments of equal size, then a very short segment, 

 and finally the xiphoid process or xiphisternum, a long, slender 

 rod, which bears behind a horizontal plate of cartilage. The first 

 pair of ribs articulate with the sides of the manubrium, and the 

 succeeding six pairs at the junctions between the segments. 



THE SKULL OF DOG 



The details of the skull of the rabbit are difficult to make out, 

 and it is much better to study first that of the dog (Figs. 337, 338), 

 on which the following description is based. The skull of verte- 

 brates has two parts, a dorsal and a ventral, but in mammals 

 the latter, w^hich is well seen in the dogfish (p. 317) is practically 

 reduced to the jaws. The dorsal part consists of a cranium or 

 brain box surrounding the brain, and two pairs of sense capsules, 



