480 VERTEBRATA. 



The first set of arches is appended to the lateral portions of 

 the cranial vertebrae, and the bones thus derived enter largely 

 into the composition of the respiratory apparatus. In Man this 

 important portion of the skeleton is reduced to a mere rudiment, 

 distinguished by the name of the os hyoides ; and in the human 

 subject its relations and connexions with the surrounding parts 

 are so obscurely visible, that the student is scarcely prepared 

 to witness the magnitude and importance of the hyoid framework 

 in other classes, or the amazing metamorphoses which, as we shall 

 afterwards see, it undergoes. 



Behind the hyoid apparatus, other arches are attached to the 

 transverse processes of the spinal vertebrae, called ribs ; and the 

 study of these appendages to the spine is one of the most in- 

 teresting points in the whole range of osteology. In Fishes, 

 wherein respiration is effected entirely by the movements of largely 

 developed hyoid bones, the ribs are mere immoveable derivations 

 from the transverse processes of the vertebrae, and serve exclusively 

 for the attachment of muscles. In Reptiles, respiration is still 

 accomplished by the os hyoides ; and the ribs, thus performing a 

 secondary office, become convertible to different uses, and assume 

 various forms and proportions. In the Amphibious Reptiles, the 

 most nearly approximated to Fishes, they either do not exist at all, 

 as being needed neither for respiration nor locomotion, or they are 

 represented by minute and almost imperceptible rudiments ap- 

 pended to the extremities of the transverse processes of the verte- 

 bras. In Serpents the ribs are wanted for locomotion, and are 

 accordingly developed from the head nearly to the tail, forming a 

 series of strong arches, articulated at one extremity with the ver- 

 tebral column by a very complete joint ; but at the opposite ex- 

 tremity they are loose and unconnected. In proportion, however, 

 as the hyoid bones, with the larynx, of which they form an impor- 

 tant part, become converted into a vocal apparatus, as they gradu- 

 ally do, the ribs assuming more complete developement in a certain 

 region of the spine, and, being augmented by the addition of a 

 sternal apparatus, form a complete thoracic cavity, and thus become 

 the basis of those movements of the body which in hot blooded 

 animals are subservient to respiration. 



The next additions required to complete the skeleton, are two 

 pairs of locomotive limbs, representing the legs and arms of Man. 

 Infinitely diversified as are these members both in form and office, 

 they are, when philosophically considered, found to be constructed 



