850 SKELETON 



of the Atlas to be mentioned presently, and the position of the facets 

 at once distinguishes the anterior from the posterior end of the bone. 

 The transverse processes articulate with the tuherculum of the corre- 

 sponding rib, while the capitidum of the latter does the like with a 

 knob or facet on the side of the anterior portion of the centrum. 

 When the vertebrae are free the centrum of each articulates with 

 that of those next to it by complicated joints, exhibiting four kinds 

 of configuration, in accordance with which the vertebra are distin- 

 guished as — 



1. Heterocoelous, or those which have saddle -shaped articular 

 facets. In them the anterior surface is concave in a transverse, 

 but convex in a vertical, direction, while the posterior surface 

 shews the condition reversed. When looked at from the ventral 

 side the joints appear to be " procoelous," but " opisthocoelous " 

 when seen from the side. This heterocoelous formation is the most 

 perfect one attained by the vertebral column, and is typical of, and 

 restricted to. Birds. There are however a few exceptional cases 

 in which the joints are not heterocoelous. 



2. Amphicodous, or those in which each end of the vertebra is 

 concave. This is the lowest condition, and is rapidly passed 

 through by recent Birds in the embryonic stage ; but Archseopteryx 

 seems to have had biconcave vertebrse of this kind, and the dorsal 

 and cervico-dorsal vertebrse of Ichthyornis were undoubtedly thus, 

 while the few well-preserved cervicals of the latter indicate transi- 

 tional steps towards the heterocoelous condition. Among recent 

 Birds the caudal vertebrse alone are occasionally more or less amphi- 

 coelous, but this may not be a primitive feature. 



3. Procoelous, or concave in front — a condition found only in the 

 Atlas. 



4. Opisthocoelous, or concave behind, so as to receive a corre- 

 sponding knob on the anterior face of the following vertebra, 

 instances of which occur in the thoracic region of the Sphenisci, and, 

 though in a much less degree, in various Steganopodes, Lari, Limicoix, 

 PsiUaci and Steatornis. 



The Procoelous and Opisthocoelous types are not to be regarded 

 as fundamentally important, as they are not primary features, but 

 produced by adaptation to functional requirements. Neither of them 

 necessarily indicates a Reptilian descent for Birds, nor can their 

 modifications be used as valid characters in determining the affinities 

 of various groups of the Class. The prevalent type among Reptiles 

 is the Procoelous, while Opisthocoelous vertebrae are common among 

 Mammals. 



The articulations of the vertebrte are further complicated by 

 the presence of a ring or pad of fibrous or cartilaginous tissue 

 interposed between the centrum of each vertebra and of that next 

 to it. These pads vary much ; when fully developed they are 



