BLOOD. 137 



especially distributed. The embryo is ultimately entirely in- 

 closed in the allantoide (the chorion of V. Baer,) and is inti- 

 mately connected with the membrane of the shell. The mutual 

 action of the allantoide and the atmosphere, takes place directly 

 through the membrane of the shell, and the shell itself, and 

 thus it may be regarded as a proper respiratory organ, whose 

 development has corresponded throughout with that of the 

 embryo. 



In birds, the lungs do not occupy the whole of the thoracic 

 cavity, but are placed in its furthest extremity: the thoracic 

 and abdominal cavities are not separated by a diaphragm. 

 Openings are situated on the surface of the lungs which admit 

 the air from those organs into the large cells situated around 

 the pericardium and between the viscera of the abdomen: the 

 air can pass from these cells even into the cavities of the bones. 



Respiration is conducted in fishes much on the same prin- 

 ciple that it is in the foetus of the mammalia. The venous 

 blood is conveyed to the gills, where it circulates in the capil- 

 laries, and absorbs oxygen and nitrogen from the air which is 

 contained in the water, and in this way it becomes arterialized. 

 Humboldt and Proven9al have carefully studied the process of 

 respiration in fishes, and have proved that they take up oxygen 

 and nitrogen from the air which is diftused through the water, 

 and that they exhale carbonic acid ; that the quantity of oxygen 

 which they absorb is more than is replaced by the carbonic acid 

 expired; that fishes absorb oxygen from boiled water which has 

 been subsequently impregnated with half its volume, but that 

 they only survive in it for a short time; and, lastly, that they 

 die in water from which the air has been removed, or in which 

 they have respired for any time. 



The water (from the Seine) in which these experiments were 

 conducted contained from -0266 to -0287 of its volume of atmos- 

 pheric air, of which from -306 to '314 was oxygen. The amount 

 of carbonic acid varied from "06 to -11 of the volume of atmos- 

 pheric air. 



The water was inclosed in bell-glasses over mercury, through 

 which the fishes w^ere introduced into it. In experiments with 

 tenches they observed, that from 100 parts of atmospheric air 

 there were abstracted 22-8, 13-6, 23-4, 15-5, 17-4, 22-8 parts, 

 the variations depending on the duration of the experiment 



