462 THE MECHANICAL FUNCTIONS. 



must necessarily pass over a multitude of instances 

 of express adaptation, which are suited only to par- 

 ticular cases, and are, consequently, of minor im- 

 portance as regards the general plans of organiza- 

 tion. In the sort of bird's-eye view that I am 

 taking of the endless modifications of structure 

 which have been executed in conformity with those 

 plans, I am only able particularly to notice such 

 as are most remarkable. 



§ 8. Rodent ia. 



As the tribes of mammalia we have hitherto ex- 

 amined employ the anterior extremities for the 

 purposes of progression only, they are destitute of a 

 clavicle. In most of those which follow, and where 

 a greater developement of the limb confers more 

 extensive and more varied powers of motion, appli- 

 cable to a greater range of objects, this bone is 

 found. In the greater number, however, it is 

 merely in a rudimental state ; that is, developed 

 only to a certain extent, one portion being bony, 

 and the rest cartilaginous ; as if the ossification 

 had been arrested at an early stage. These imper- 

 fect clavicles are too short to connect the scapula 

 with the sternum ; the rest of the space being eked 

 out by cartilage, and by ligaments : but still they 

 are of great use in affording points of attachment 

 to the muscles of the limb, and giving them the 

 advantage of acting by a rigid lever. The carni- 

 vorous tribes, which make considerable use of their 

 fore paws in striking and seizing their prey, have 

 clavicles of this description. Those quadrupeds 



