280 THE MECHANICAL FUNCTIONS. 



them first in their more perfectly developed forms, as they 

 are presented in the higher classes of quadrupeds. In the 

 following descriptions, the skeleton of the Hog (Fig. 181) will 

 be taken for the purpose of reference. 



The ribs consist of arches of bone affixed at their upper 

 ends to the bodies of the vertebrae, and also, by a separate 

 articulation, to their transverse processes ; where, in general, 

 they are allowed a slight degree of motion. Their primary 

 use is to defend the vital organs situated in the region of the 

 chest, or thorax, (namely, the heart and the lungs ;) but they 

 are subservient also to the function of respiration, by the al- 

 ternate movements that are given to them by their mus- 

 cles. The two parts, of which they are composed, often form 

 an angle by their junction, and at this angle a process occa- 

 sionally extends, for the purpose of forming connexions with 

 the neighbouring ribs. 



The ribs are connected in front with the breast bone, or 

 sternum (s,) often by the intervention of cartilages, which, 

 from their similarity of form to the ribs, appear as continua- 

 tions of them, and are provided apparently to eke out the re- 

 mainder of the semicircle. These cartilages, which have 

 been termed the sterno-costal appendices, often become ossified 

 either wholly or in part. 



The sternum is formed of nine elementary pieces, each pro- 

 ceeding from a separate centre of ossification. Two of these 

 occupy the end which is nearest to the head, four are lateral, 

 and two are situated at the opposite extremity : one only be- 

 ing central and surrounded by the rest. Few subjects in 

 comparative osteology are more curious and instructive than 

 to trace the development of these several elementary parts 

 in the different classes of animals, from the rudimental states 

 of this bone as it occurs in fishes, to its greatly expanded con- 

 ditions in the tortoise and the bird, wdiich severally exhibit 

 the most opposite proportions of these elements. 



Last in the order of constancy come the bones of the ex- 

 tremities. As we ascend in the scale of animals we may 

 observe the prevalence of a tendency to the concentration 

 of organs, and consequently to the diminution of their num- 



