612 VEBTEBRATA. 



tyro knows, 1st, the dorsal vertebrae ; 2ndly, the ribs, with their carti- 

 lages ; and 3rdly, the sternum. But it is not in Man that we must 

 expect a perfectly-developed thoracic framework ; it is in the Birds, which 

 are destined to rise in the air by the assistance of their proportionately- 

 powerful thoracic extremities. If, therefore, we examine the thorax of 

 a Bird, we find it composed of pieces which in Man are absolutely 

 wanting : we see, 1st, the vertebrae ; 2ndly, the dorsal ribs, firmly arti- 

 culated on each side both with their bodies and transverse processes ; 

 3rdly, the sternal ribs, extending from the ribs last mentioned to the 

 sternum ; and lastly, the sternum, composed, as we shall afterwards 

 see, of various elements not found in the human body. If we 

 prosecute our survey a little further, we shall find this portion of the 

 skeleton offering the greatest possible variety as regards the presence or 

 absence of the elements above enumerated : thus, in the Frog we have 

 vertebrae and sternum, but no ribs ; in the Serpent, vertebrae and dorsal 

 ribs, but no sternum or sternal ribs ; in Man the sternal ribs are repre- 

 sented by the costal cartilages ; and thus a thorax of every required 

 description is constructed by adding or taking away, expanding or con- 

 tracting certain elements, all of which a typical skeleton might be sup- 

 posed to contain developed in a medium condition. 



(1637.) Comparison of the skeleton of a Fish with those of the higher 

 animals demonstrates that the natural arrangement of the parts of the 

 endoskeleton is in a series of segments succeeding each other in the axis 

 of the body. These segments are not, indeed, composed of the same 

 number of bones in any class, or throughout any individual animal ; 

 but certain parts of each segment do maintain such constancy in their 

 existence, relation, position, and offices, as to enforce the conviction 

 that they are homologous parts, both in the constituent series of the 

 same individual skeleton, and throughout the series of vertebrate animals. 

 Each of these primary segments of the skeleton is designated a " ver- 

 tebra," but with as little reference to the primary signification of the 

 word as when the comparative anatomist speaks of a sacral vertebra. 

 A vertebra is defined by Professor Owen as " one of those segments of the 

 endoskeleton which constitute the axis of the body and the protecting 

 canals of the nervous and vascular trunks ; " such a segment may also 

 support diverging appendages. 



(1638.) A vertebra consists, in its typical completeness, of the ele- 

 ments or parts represented in the following diagram : 



