THE SKELETON. 



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vascular ; tliey grow by the addition of layers to their circumference, 

 or they may be cast off when too small for the growing body, and be 

 reproduced of a more conformable size ; but they have no inherent 

 power of repair. 



The internal bones of the Vertebrata are vascular ; they grow by 

 internal molecular addition and change, and have the power of repair- 

 ing fracture or other injury. 



Such are the broad and obvious distinctive characters of the 

 skeletons of the Invertebrate and Vertebrate animals ; the contrasts 

 having relation chiefly to the difference in the development of the 

 nervous system. Thus, when the powers of discerning and avoiding 

 lethal or hurtful agencies are dull and contracted, the entire animal 

 is protected by a hard insensible dermal armour, or exo-skeleton ; 

 but, as those powers become expanded and quickened, the body is 

 disencumbered of its coat of mail, the skeleton is put inside, and made 

 subservient to the activities, and the skin becomes proportionally 

 more susceptible of outward impressions of pleasure and pain. 



Some estimable anatomists, who have more especially devoted their 

 attention to the detection of the corresponding parts in difierent ani- 

 mals, have supposed that these different functions were performed by 

 modifications of essentially the same or homologous parts of the 

 skeleton. 



^^^ 



Segment of exo-skeleton, Astacus. 



Segment of cndo-skeleton, Mainmal. 



Observing that a segment of the outer skeleton of an articulate 

 animal, the thoracic ring of a lobster for example {Jig- 5.), formed 

 a small canal (e s) for the nervous trunks, and a larger one (Ji) for 

 the vascular trunks and plastic organs ; and that a thoracic segment 

 of the skeleton of a Vertebrate animal {Jig. 6.) also formed a small 

 protecting canal for the spinal chord {c, ns), and a larger hoop (c, hs) 

 about the vascular and other viscera of that cavity, — they have con- 

 cluded that both were modifications of the same elements or primary 

 segment of the skeleton. Carus, for instance (No. i. p. 73.), calls 

 both rings " vertebrae ; " and Geoffroy St. Hilaire (ii. p. 119. pi. 7.) 

 thought it needed but to reverse the position of the Crustacean, — to 

 turn what had been wrongly deemed the belly upwards, — in order 



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