374 



THE ANIMAL KINGDOM. 



neck is long and mobile, and contains a variable number of vertebrae. The 

 centra of the praesacral vertebrae are procoelous, cylindroidal and saddle- 

 shaped posteriorly. They articulate by synovial joints with a cartilaginous 

 meniscus interposed between successive centra. Certain of the dorsal 

 vertebrae are opisthocoelous in the Penguins and Auks, and all the ver- 

 tebrae are amphicoelous in the extinct Archaeopteryx and Ichthyornis. 

 Some of the dorsal vertebrae are occasionally anchylosed and they always 

 possess strong spines and ligaments. The terminal 4-6 caudals fuse as a 

 rule and form a ploughshare bone or pygostyle. The cervical ribs are 

 small, provided with double articulations, and anchylosed to the vertebrae 

 with the exception of the 2-3 last which are large and free. The dorsal 

 ribs are divided into a vertebral and sternal section, both well ossified. 

 The posterior ribs fail to reach the sternum. Ossified plates or ' processus 

 uncinati ' are attached to the posterior edges of certain of the vertebral 

 sections of the dorsal ribs. The sternum is very large, convex ventrally, 

 and the original cartilage replaced by membrane bone. It has in the 

 majority of living birds, in Archaeopteryx and Ichthyornis, a prominent 

 ventral keel, probably derived from the posterior part of the interclavicle : 

 hence Carinatae. In a few living birds and the extinct Hesperornis this 

 keel is absent and the sternum smooth and raft-shaped : hence Ratitae. 

 The scapula is thin, narrow, sabre-shaped, either anchylosed to the coracoid 

 (Ratitae) or united to it by ligament (Carinatae). The ventral ends of the 

 coracoids are received into grooves of the ventral surface of the sternum. 

 The clavicles are rarely absent (most Ratitae) and are generally fused at 

 their lower extremities, forming the furcula. The interclavicle is present 

 apparently in development : its anterior end either becoming entirely 

 fibrous, or else partially ossified as hypocleidium and fused to the furcula, 

 and its posterior either remaining fibrous (Ratitae) or ossified as the keel of 

 the sternum (Carinatae). The pubes form a ventral symphysis in the 

 Ostrich (Struthio\ and the ischia a dorsal symphysis in Rhea. The con- 

 formation of the limb-bones is characteristic. The fibula has a pointed 

 lower extremity and is shorter in the adult than the tibia. The scaphoid 

 is present in the carpus ; the lunar and cuneiform fused. The hand has but 

 three digits, first, second, and third, with fused metacarpals (except in 

 Archaeopteryx) : the foot, four digits, the fifth toe being always absent. In 

 Struthio the third and fourth are alone present, but the tarso-metatarsus 

 retains a trace of the articulation for the second toe. In Aves as in Lacer- 

 tilia the phalanges of the toes increase typically in number from the first to 

 the fourth toe in the series 2, 3, 4, 5. 



The cerebral hemispheres are large and made up chiefly of corpus 

 striatum. They touch the cerebellum posteriorly, and the two solid optic 

 lobes are thus thrust aside laterally. The olfactory lobes are small. The 

 cerebellum has a median foliate lobe, showing in longitudinal section an 



