38 ANATOMY OF VERTEBRATES. 



has a capsule and synovial fluid, and the neural spine is shortest 

 in the sixth, where the tail has the greatest extent of motion ver- 

 tically, the transverse bend being checked by the size and length 

 of the transverse processes. The neural spines can be brought 

 by dorsal inflection into contact Avith the sacrum ; and in this mo- 

 tion the side-muscles, which at first tend rather to oppose the 

 elevators, become, as the motion proceeds, themselves elevators, 

 and complete it by a jerk ; this throwing up the tail upon the 

 back, as if operated on by a spring, is a conspicuous characteristic 

 of the living bird.^ In most birds of flight the caudal series are 

 habitually curved upward, as in fig. 23 ; in the few birds that can- 

 not fly the tail is straight, and the terminal centrum is not ex- 

 panded. In the Apteryx there are nine caudal vertebrae, which are 

 deeper, and project farther below the posterior portions of the iliac 

 bones than in the other birds : as they recede, they increase in 

 lateral and diminish in vertical extent ; the spinal canal is conti- 

 nued through the first five, and they are all moveable upon each 

 other, excepting the last two, homologous with the expanded ter- 

 minal mass in other birds, but which here exceeds the rest only in 

 its greater length, and gradually diminishes to an obtuse point. 

 In the Ostrich the coiTesponding vertebra is expanded for the 

 support of the caudal plumes, but in the Apteryx it offers the 

 same inconspicuous developement as in the Rhea and Emeu. In 

 the ' rumpless ' breed of domestic fowl the coccyx is reduced to 

 a single stumpy bone. In some bre^dpennate sea-birds I have 

 found as many as eleven free caudal vertebra3 ; only in the extinct 

 Archeopteryx of the upper oolitic period was the tail a conspicuous 

 appendage to the trunk, formed by about twenty elongate ver- 

 tebrae, each of which supported a pair of small and slender quill- 

 feathers. 



The terminal vertebrae, ungrasped by the pelvis in the embryo 

 bird, may equal in number those of the ancient feathered fossil ; 

 and if such vertebras participated in the ratio of growth of other 

 parts of the skeleton, without subsequent stunting and con- 

 fluence, they would more or less repeat the strange and unique 

 feature in the skeleton of Archeopteryx ; but the metamorphosis 

 of the tail which has taken place in the bird's skeleton in the 

 transition from the mesozoic to the neozoic life-periods of the 

 class, is analogous to that from the protocercal to the homocercal 

 type of tail, which marks the progress in fishes from the palaeo- 

 zoic to the mesozoic periods.''^ 



' XX-. 2 XV. p. 45, pis. i. and iii. 



