VEETEBEiE OF VEETEBEATA. 435 



aud Crocodiling as well as in Aves, the last ribs of the cervical 

 region differ but little in length from the succeeding ones, which 

 are connected with the sternum. The same applies to the lumbar 

 region in the Saurii ; in Birds this region is united to the true 

 sacral portion. The sacral portion of the column is increased in 

 size, inasmuch as in Reptiles there is, at the least, a second vertebra 

 (Fig. 227, a b) in addition to the one found in the Amphibia. These 

 vertebras attain firmer connections, and are completely fused in 

 Birds, where a Large number of pre-sacral and post-sacral ver- 

 tebras are attached to the primitive sacral vertebras (Fig. 228, 

 a b), and the whole set is united to the ilium. In the so-called 

 sacrum of Birds, thoracic as well as lumbar and caudal 

 vertebras may be recognised; in the Struthiones the whole 

 number is as great as twenty-three. The two true sacral vertebras 

 are very distinct in the Gallinas, many Natatores, and in birds of 



P re F- 



The caudal region is the one which varies most in character ; in 



the Chelonii and Aves it is considerably reduced. 



In Carinate Birds, the caudal vertebras are not only reduced in 

 number, but four to six of them, which were separate in the embryo, 

 are fused together; and the bone thus formed, which is of some 

 size, and is ordinarily known as the " ploughshare-shaped bone," 

 forms the terminal portion of the vertebral column ; it is generally 

 produced into an upright plate, in adaptation to its relations to the 

 rectrices feathers. 



§ 333. 



In the Mammalia the cartilaginous rudiment of the vertebral 

 column grows around the chorda dorsalis, and is constricted at 

 points, each of which corresponds to a centrum ; the notochord is 

 therefore retained for some time between the vertebras (Fig. 222, E). 

 The intervertebral circular disc is developed out of the surrounding 

 tissue, and the remnant of the notochord is preserved in it, under 

 the form of the " gelatinous nucleus." The cartilage is continued 

 from the centra into the superior arches. Independent ossifications 

 appear in the centrum as well as in the arches, and the bony pieces 

 thus formed do not fuse until growth is finished. 



On most vertebras the arches develop spinous processes. In the 

 long-necked Ungulatas (Giraffe, Camel, Horse), they are not found 

 on the cervical vertebras, but are greatly developed on the dorsal 

 ones. This holds also for the Cetacea, where the caudal region is 

 also of great size. As a rule articular processes are developed; they 

 have undergone degeneration in the Cetacea only. We are in the habit 

 of calling all kinds of structures transverse processes, whether they 

 spring from the arches or from the centra. These processes are 

 simpler in the cervical and thoracic regions. In the former they 

 are somewhat complicated by being fused with rudimentary ribs, 

 which unite with them and aid in the formation of a foramen trans- 

 versarium. They likewise carry ribs in the thorax, and here they 



2 f 2 



