Chap. VIII.] THE SKELETON. 167 



present are undeveloped ; and (3) Metamorphosis, where there is 

 a notable departure from the typical form. The student is, 

 however, to bear in mind that the typical form has not neces- 

 sarily any existence as such in nature, but is a conception of the 

 human mind. To explain : photographs are now taken which 

 represent, e.g. the typical criminal. A number of photographs 

 of representative criminals are separately taken, and then the 

 images of all are combined on one plate. The result is a typical 

 photograph which resembles no one individual, but represents 

 all. So too the ideal vertebra resembles no one individual ver- 

 tebra, but represents all. If, therefore, we speak of a particular 

 vertebra as typical, we mean that its divergence from the ideal 

 type is comparatively slight. 



The Ribs and Sternum. (1.) In the 'Rabbit. Free ribs are 

 connected with and characterise the thoracic vertebrae. There 

 are seven true ribs which, with the sternebrae below, constitute 

 inferior or haemal arches. Behind these are five false ribs, in- 

 complete below. Each true rib consists of a dorsal osseous 

 portion, the vertebral rib, and a ventral cartilaginous portion, 

 the sternal rib. The vertebral rib has a head (capitulum), which 

 articulates with the capitular facets of the vertebrae, and a 

 tubercle on the dorsal side, which articulates with th,e> tubercular 

 facet on the transverse process of the vertebra. Just external 

 to the tubercle is a dorsal process. 



The sternum consists of six segments or sternebrce, of which 

 the first, manubrium, is large and keeled ventrally. The last has 

 a rounded plate of cartilage (the xiphoid process). Seven pairs of 

 ribs articulate with the sternum, the first two pairs with the manu- 

 brium, the others with the interspaces between the sternebrae. 



(2.) In the Fowl there are five complete or true ribs. Each 

 has a capitulum and tuberculum, and is divided by a transverse 

 articulation into a dorsal (vertebral) and a ventral (sternal) rib, 

 both of which are osseous. At their junction they form an open 

 angle, which is widened during the process of respiration. Each 

 vertebral rib has a backward and upward process (the unci- 

 nate pr.). 



