THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 75 



3. The ribs. The ribs of mammals consist of a dorsal, bony part, the ver- 

 tebral rib, and a ventral cartilaginous portion, the sternal rib or costal carti- 

 lage. The vertebral rib is articulated to the vertebra by two heads, an upper 

 tuberculum, whose facet engages the costal facet on the under surface of the 

 transverse process; and the lower capitulum, attached between two centra to 

 demifacets, i.e., half a facet on the posterior end of one centrum and half a facet 

 on the anterior end of the succeeding centrum. The tuberculum is diminished 

 on the more posterior ribs and wanting on the last three ribs which are provided 

 with capitular heads only. The narrow part of a rib between tuberculum and 

 capitulum is called the neck; the point of greatest curvature, just a little beyond 

 the tuberculum, the angle; the remainder of the rib, the body. Those ribs which 

 reach to the median ventral line and are independently attached to the median 

 ventral structure (the breastbone) are known as true ribs; those which are not 

 so attached to the breastbone are called false ribs; and those false ribs whose 

 ventral ends are free are floating ribs. 1 The cat has nine true ribs and four 

 false ribs, of which the last one is floating; the rabbit has seven true ribs and 

 five false ribs, of which the last three are floating; in man there are seven true 

 ribs and five false ribs, the last two floating. The ribs of mammals are inter- 

 muscular ribs. Draw one of the typical thoracic vertebrae with its ribs, from 

 the front. 



4. The lumbar vertebrae. There are seven lumbar vertebrae in the cat 

 and rabbit. They are large and stout vertebrae with prominent neural spines, 

 conspicuous zygapophyses, and long transverse processes directed craniad. 

 Below the postzygapophysis is a pointed projection, directed caudally, called 

 the accessory process. Draw a lumbar vertebra from the side. 



5. The sacrum. The sacrum is composed of a variable number of vertebrae 

 fused together for articulation with the hind limbs. There are three sacral 

 vertebrae in the cat, generally four in the rabbit (of which only the first two really 

 contribute to the attachment), five in man. The boundaries between the fused 

 sacral vertebrae are readily made out by means of the openings between them 

 through which the spinal nerves pass out, and by means of the number of neural 

 spines, zygapophyses, etc. The first sacral vertebra assumes the greater part 

 of the task of transmitting the support of the hind limbs to the vertebral column; 

 for this purpose it has large lateral expansions bearing articular surfaces for the 

 insertion of the bony structure which supports the hind limb. These lateral 

 expansions consist in part of transverse processes and in part of sacral ribs, 

 indistinguishably fused to the vertebra. 



6. The caudal vertebrae. The caudal vertebrae are variable in number 

 in mammals. Neural arches, transverse processes, and zygapophyses diminish 



1 This use of the terms true and false should not be confused with the usage previously given in 

 which the expression true rib refers to intermuscular ribs and false rib to subperitoneal ribs. In this 

 comparative sense all mammalian ribs are "true" ribs. 



