MAMMALIA 267 



for the ophthalmic and maxillary branches of V occurring in 

 front of the alisphenoid, and that for the mandibular branch 

 of V behind it. 



The skeleton of the limbs is preceded with few exceptions 

 in cartilage. The limbs are developed, as in the chick, from 

 a horizontal fold on each side of the body, known as the 

 Wolffian fold. The fold extends from behind the branchial 

 region to the end of the region of the body cavity. The middle 

 part of the fold on each side disappears, and the remaining two 

 parts contract and grow outwards, receiving mesoderm from 

 the somatopleure and muscles from buds of the myotomes. 

 The mesenchyme gives rise to procartilage, and then cartilage. 

 The cartilages are disposed in segments corresponding to 

 the segmentation of the limb, and can be identified as the 

 precursors of the various elements of the skeleton. The 

 cartilages grow with the limb, and later become ossified. The 

 outer layer becomes converted into periosteum and the cartilage 

 is invaded by its outgrowths. The result is that a new circum- 

 ferential growth takes place, formed by fresh layers derived 

 from the periosteum ; and as the bone increases in width it is 

 absorbed internally by cells, called osteoclasts. A marrow 

 cavity is thus formed in the place occupied by the original 

 cartilaginous model of the bone. This process continues during 

 the growing years, and after birth the bone obtains processes 

 by the mechanical conditions of movement and support. At 

 the same time it is growing in length. This is secured by the 

 bones being developed in three portions : the middle part, the 

 shaft or diaphysis, and the two ends or epiphyses. Between 

 these bony elements the cartilage persists and grows, being 

 converted into bone above and below. After growth has been 

 accomplished, the cartilage is finally replaced by bone. 



The pectoral girdle of the rabbit consists of a scapula or 

 shoulder-blade and a rudimentary clavicle or collar-bone on 

 each side. The latter is often absent in mammals, but it is 

 preserved in a perfect condition in the Primates, and in similar 

 cases where the movements of the fore-limbs are not reduced 

 to a mere hinge-like state. 



The triangular-shaped scapula is suspended by muscles, 

 and is provided with a prominent spine externally associated 



