591 



CHAPTER XXIX. 



AVES BIRDS. 



(657.) THE class of Vertebrate animals which now offers itself to 

 our notice contrasts remarkably with the cold-blooded and apathetic 

 inhabitants of the water ; and even with the slow-moving Reptile, 

 that languidly crawls upon the surface of the ground, or drags on 

 an amphibious existence in the marsh or on the shore. The Bird, 

 ordained to soar into the regions of the air, and not only to sustain 

 itself in that thin medium but to skim from place to place with 

 astonishing rapidity, needs a strength of muscle and activity of 

 limb even greater than that conferred upon the mammiferous qua- 

 druped. Senses of the utmost acuteness are now requisite, com- 

 bined with instinct and intelligence of a high order ; and accord- 

 ingly, both as regards their faculties and enjoyments, the feathered 

 tribes far surpass the other oviparous Vertebrata. 



Next to that improvement in the condition of the nervous 

 system, which we have all along been able to trace advancing 

 part passu with the increase of sagacity and the expansion 

 of the bodily faculties, the most remarkable circumstance observ- 

 able in the economy of Birds is the elevated temperature of their 

 bodies and the heat of their circulating fluids. In the Reptile an 

 impure and semi-oxigenized blood was slowly propelled through 

 the system from the undivided ventricle of their trilocular heart ; 

 and we found their energies, their instincts, and their affections 

 proportionately feeble and obtuse : but now, not only does the 

 heart become divided into four cavities, one ventricle being appro- 

 priated to transmit venous blood to the lungs, while the other 

 drives a pure and highly arterialized fluid in copious gushes to the 

 remotest regions of the body; but, as though even this was not suf- 

 ficient tomeet the necessities of the case, the whole interior of the 

 bird is permeated by the atmospheric air, which penetrates even 

 into the bones ; and the respiratory function being thus rendered as 

 complete as possible, all parts of the muscular system are abun- 

 dantly supplied with blood arterialized to the utmost, and every 

 fibre, quivering with life intense, is ready to exert that vigorous 



