All Introduction to Zoology. 35 



thera is also observed the fitst trace of a regular organ of hearing, and they 

 are in many respects very highly advanced. 



In the fishes, the only circumstance to be observed with respect to their 

 circulation is, that they have only a single or simple heart j and this prime 

 mover of the system is placed between the great venous trunlis of the body, 

 the venae cavae, and the artery leading to the gills ; so that this is a purely 

 branchial heart. By the branchial veins the blood is re-collected into one 

 common trunk, tlie prolongation of which, however, must be regarded as an 

 aorta ; for although no second muscular heart intervenes in these animals to 

 accelerate the circulation, the ramifications into which the vessel divides and 

 subdivides, diffuse the blood through the system, by virtue of the impetus 

 communicated by the mere contractile force of the vessels themselves. 



Reptiles possess also a single heart, but in them the main artery im- 

 mediately divides j one branch passing off, as the great pulmonary artery, 

 to ramify in the lungs ; and the other, as the aorta, being destined for the 

 supply of the system. Thus therefore in these animals a fractional part only 

 of the blood is made to undergo, at every round, the process of aeration. 



In the mammalia the heart is double, a distinct pulmonary and an aortal 

 heart being associated in juxta-position as a compound organ, of which the 

 former occupies the right, and the latter the left side. Here the venae 

 cavae convey the collective blood of the whole system, mingled with the 

 chyle previously absorbed by the lacteals from the intestines, into the right 

 auricle, through the valve of which it passes into the right ventricle, thence 

 to be propelled through the pulmonary artery into the lungs. From the 

 lungs the aerated blood descends by the pulmonary veins into the left 

 auricle, and flows through its valve into the left ventricle, which, by the 

 firm contraction of its powerful muscular walls, urges it through the main 

 conduit of the aorta, on its course through the whole system. 



There are many beautiful contrivances subsidiary to the ends of the cir- 

 culation, into the mention of which we cannot now enter j they must form 

 the subject of future communications. 



We liave thus traced the whole process of the preparation, aeration, and 

 diffusion, of the nutritive fluid. How this homogeneous fluid becomes 

 assimilated and converted into such various and dissimilar substances, 

 depositing in one place calcareous phosphates and carbonates, as bone and 

 shell ; in another, fibrine and gelatine, for the membranous and fleshy 

 textures ; or why, being conveyed into one gland, it should issue in a fluid, 

 such as bile j or in another, assume the totally opposite character of milk j 

 is an inscrutable mystery, and most probably will ever remain so. These 

 arc pha-nomena which can only be simply stated ; they admit not explana- 

 tion J they are part of — 



■-^—— " Nature's chemistry, 

 By man's weak art inimitable." 



by his limited faculties altogether incomprehensible. 



