COMMON PIGEON. 59 



sacral, and a caudal region. The articular surfaces of the centra are 

 typically procoelous and cylindroidal, i. e. concave from side to side and 

 convex from above downwards anteriorly, curvatures which are reversed 

 posteriorly. The cervical and dorsal vertebrae have synovial joints. A 

 ring of fibres binds together the edges of the opposing surfaces. Between 

 them is interposed a fibre-cartilaginous meniscus thick at its circumference, 

 thin centrally where it is perforated for the passage of the ' suspensory 

 ligament ' which unites the vertebral centra. The free caudal vertebrae 

 have usually flattish centra. They articulate as in Mammalia by inter- 

 vertebral discs (annuli fibrosi), which have centrally a ' nucleus pulposus/ 

 the homologue of the suspensory ligament and formed as it is from 

 notochordal cartilage. 



The length of the neck in every Bird is at least equal to the height 

 from the ground at which the legs carry the body and to the distance from 

 the root of the neck to the last caudal vertebra. The actual length varies, 

 and depends chiefly on the number of vertebrae present, and not on the length 

 of their centra. In Mammalia while the number of vertebrae is nearly 

 invariable, the length of their centra is very variable. The atlas is ring-like 

 and articulates with the occipital condyle by a deep cup completed as in 

 Reptilia by the odontoid process of the axis. This latter vertebra^ has 

 a neural spine, and an inferior spine, a structure present also in the two 

 next vertebrae. The odontoid is anchylosed to its centrum. The third 

 vertebra has its neural arch deeply emarginated before and behind, a 

 peculiarity repeated in the seven following vertebrae. It has also, like the 

 next nine vertebrae, cervical ribs anchylosed to the superior and inferior 

 transverse processes, and inclosing a canal which lodges the vertebral 

 artery and vein with the main trunk of the sympathetic. The fourth 

 vertebra has no neural spine, and its centrum has ventrally a pair of down- 

 growths which form a demi-canal for the protection of the common carotid 

 arteries. These features are repeated in the succeeding vertebrae to the 

 tenth inclusive. The eleventh and twelfth have inferior spines ; the two 

 following the thirteenth and fourteenth have neural spines ; their ribs are 

 free but not connected to the sternum, and the last pair carry recurrent 

 or uncinate processes like the first four pairs of dorsal ribs. By some 

 anatomists the thirteenth and fourteenth vertebrae are counted as dorsal. 

 Five vertebrae make up the dorsal region defined by the presence of free 

 ribs connected to the sternum. They have large neural spines and 

 transverse processes with keel-shaped centra. The three first have their 

 centra anchylosed, and the ligaments connecting their neural spines, 

 inferior spines, and transverse processes ossified. The fourth dorsal is free : 

 the fifth unites with the sacrum. The sacrum contains, as it always does 

 in Birds, vertebrae derived from four regions. The first is a dorsal vertebra 

 with ribs. The six vertebrae succeeding it are lumbar, of which the three 



