CIRCULATORY SYSTEM OF BIRDS. 355 



whea so employed. 2. That tlio c|u;iiitity of air tlius introcUiced penetrates 

 the most iuternal recesses of their bodies, tending to dry the marrow in tlie 

 bones, and a portion of the fluids. A diminution of specific gravity is the 

 result, tiie cause of which has been, iu his opinion, vainly sought in the 

 quantity alone of permeating air. 



3. That iu birds tlie oxidation of the nourishing juices is not entirely 

 effected iu the lungs, but is much promoted also in the pneumatic bags 

 above-mentioned, for their contained air operates through the membranes 

 upon tiie blood vessels and lymphatics in contact witli them : a more com- 

 plete and speedy oxidation is tlie result. 4. Tliat not only the skeleton, 

 but all the viscera are much more permeable by air iu birds than in any of 

 the other vertebrated animals. 5. That tiie air reservoirs are not always 

 symmetrical, their shape and extent depending entirely upon the form and 

 situation of the organs among wliicii they occur ; but the supply is so modi- 

 fied, that the total quantity received into tlie pneumatic bags on tlie riglit 

 side of the body is equal to that which enters into those on the left; and, 

 indeed, without the maintenance of this condition the act of flying would be 

 impossible, and that of walking difficult. 0. That no portion of a bird's 

 structure is impervious to air. It reaches even the last joints (piialangcs) 

 of the wings and feet, and the last caudal vertebra, or rump-bones. The 

 quill of the feathers is not excepted, as has sometimes been asserted. 

 7. That the air within the head has a separate circulation, and docs not 

 directly communicate with the air pipes of the rest of the body. 8. Tiiat in 

 no instance does the air come into direct contact witli tiie viscera, or nour- 

 ishing juices, but invariably through the medium of a membrane, however 

 fine and transjiarent. 9. Tiiat the volume of air wliich birds can tlius in- 

 troduce into their bodies, and the force with which tliey can expel it, oiler 

 the only explanation how so small a creature as a singing bird (the nightin- 

 gale, for example) is alile to utter notes so powerful, and, without any 

 apparent fatigue, to warble so long and so musically. 



A writer on the economy of birds, treats of their circulatory and respiratory 

 organs, as follows: "The organs of circulation and respiration in birds are 

 adapted to tlieir peculiar mode of life. They are not, however, separated 

 from the abdominal cavity by a diaphragm, as in the mammalia. The heart 

 consists of four distinctly separated cavities, — two auricles and two ven- 

 tricles, — so that the venous and arterial lilood can never mix in that organ, 

 and the whole of the blood returned from the different parts of the body 

 passes throui'h the lun!is before beini>- a"ain driven into the systemic arteries. 

 Tlie blood is received from the veins of the body in tlie riglit auricle, from 

 which it passes through a valvular opening into the right ventricle, and is 

 thence driven into the lungs. From these organs it returns through the 



