614 VERTEBKATA. 



calation of the haemal spine (fig. 307) between the ends of the haemapo- 

 physes. And this spine is here sometimes as widely expanded (in the 

 thorax of Birds and Chelonians for example) as is the neural spine 

 (parietal bone or bones) of the middle cranial vertebra of Mammals. In 

 both cases also it maybe developed from two lateral halves ; and a bony 

 intermuscular crest may be extended from the mid-line, as in the skull 

 of the Hyena and the breast-bone of Birds. 



(1642.) The ossified parts of the abdominal vertebrae of osseous 

 Pishes answer to the centrum, the neurapophyses, the neural spine, the 

 parapophyses, the pleurapophyses, and certain appendages to be here- 

 after noticed. 



(1643.) In the air-breathing Vertebrata, in which the heart and 

 breathing-organs are transferred backwards to the trunk, the corre- 

 sponding osseous segments of the skeleton are in most instances de- 

 veloped in their typical completeness, in order to encompass and protect 

 those organs. The thoracic haemapophyses in the Crocodiles are par- 

 tially ossified, and in Birds completely so, in which class the haemal 

 spines of the thorax coalesce together, become much expanded laterally, 

 and usually develope a median crest downwards, to increase the sur- 

 face of attachment for the great muscles of flight. This speciality is 

 indicated by the name " sternum," applied to the confluent elements in 

 question. 



(1644.) The typical thoracic vertebrae in Birds support diverging ap- 

 pendages, either anchylosed, as in most, or articulated, as in the Penguin 

 and Apteryx, to the posterior border of the pleurapophysis. The func- 

 tion of such appendages in this form of typical vertebra is to connect 

 one haemal arch with the next in succession, so as to associate the two in 

 action, and to give firmness and strength to the whole thoracic frame. 



(1645.) The diverging appendages are, as might be expected, of all the 

 elements of the vertebral segment, the least constant in regard to their 

 existence, and the subjects of the greatest amount and variety of modi- 

 fication. Simple, slender spines or styles in Pishes (fig. 311, 13) simple 

 plates retaining long their cartilaginous condition in Crocodiles short, 

 flat, slightly-curved pieces in most Birds, such, with one exception, is 

 the range of the variety of form to which these parts are subject in the 

 segments of the trunk. But that exception is a remarkable one, inas- 

 much as we are enabled to trace the diverging appendage of that ver- 

 tebral segment of the body, which from its form and character consti- 

 tutes the pelvic arch, through various progressive phases of development 

 from that of a simple, articulated, solitary ray, such as exists in the 

 Lepidosiren, through innumerable modifications, whereby it is adapted 

 for swimming, steering, balancing, and anchoring for exploration, for 

 burrowing, creeping, walking, and running for leaping, seizing, climb- 

 ing, or sustaining erect the entire frame of the animal under the 

 general appellation of the posterior or pelvic limb. 



