VERTEBRATED ANIMALS. 255 



work, or skeleton, which sustains and protects their fabric. 

 The quadruped, the bird, tlie tortoise, the serpent, and the 

 fish, however they may differ in subordinate details of or- 

 ganization, are yet constructed upon one uniform principle, 

 and appear like varied copies from the same original model. 

 In no instance do they present structures which are altoge- 

 ther isolated, or can be regarded as the results of separate 

 and independent formations. 



In proceeding from the contemplation of the structures of 

 articulated to those of vertebrated animals, we appear to pass, 

 by a rapid excursive flight, from one great continent to ano- 

 ther, separated by an immense gulf, containing no interme- 

 diate islands from which we might gather indications of these 

 tracts of land having been originally connected. At the 

 very first sight, indeed, the general fabrics of these two de- 

 scriptions of animals appear to have been constructed upon 

 opposite principles; for, in the one, as we have already seen, 

 the softer parts are internal, and are enclosed in a solid crust, 

 or shell, or horny covering, answering, at once, the purposes 

 of protection and mechanical support, and furnishing exten- 

 sive surfaces for the attachment of the orjrans of motion. 

 But, in the Vertebrata, the solid frame-work which serves 

 these purposes, occupies, for the most part, an internal situa- 

 tion, constituting a true-jointed skeleton, which is surround- 

 ed by the softer organs, and to which the muscles, destined 

 to move their several parts, are attached. The office of ex- 

 ternal defence is intrusted solely to the integuments, and 

 their different appendages. Such is the general character of 

 the arrangements which nature has here ado])ted; from 

 which, however, she has occasionally deviated with respect 

 to some important organs of extremely delicate texture, and 

 which require to be shielded from the slightest pressure. 

 This occurs with regard to the brain, and the spinal marrow, 

 which, we shall presently find, are specially guarded by a 

 bony structure, enclosing them on every side, and forming 

 an impenetrable case for their protection. The solid mass 

 of bone, thus provided to defend the brain, gives also the 

 opportunity of lodging safely the delicate apparatus subscr- 



