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that of the mammalia and birds ; the blood returning from the body, having cir- 

 culated through it, i9 received first by the right auricle (a), and propelled from it 

 into the right ventricle (c) ; but instead of being thrown from this cavity into 

 the lungs completely, as it is in the circulation of all warm-blooded animals, we 

 find that a portion only is distributed to these organs. The chief peculiarity in 

 the heart is met with in this cavity, for from it all the blood-vessels of both lungs 

 and body arise. From the extreme right of the cavity is sent off the aorta, or 

 rather the aortae, for the vessel immediately divides into three, the centre one of 

 which is distributed to the upper or fore limbs, head, and neck, whilst the two 

 outer ones unite into one to supply the lower, or posterior, half of the body with 

 blood. The blood sent to the lungs is received back into the left ventricle (d), 

 and thence passed, through an opening between these two cavities, into the right 

 ventricle. The course of the circulation is explained by the diagram, Fig. 4. 



Fig. 3. 



Fig. 4. 



Fig. 3 A diagram, representing the course of the blood when the animal is under 



water. The blood entering through the veins into the right auricle at b, passes di- 

 rectly into the corresponding ventricle, and is propelled through the aorta ( d) ; thus 

 following a single course, returning from the body, and immediately re-sent to it 

 without passing through the lungs. 



Fig. 4. — A diagram, representing the double or mixed circulation, when the animal re- 

 spires atmospheric air. The blood returning from the lungs, through the pulmonary 

 veins (a), into the left auricle, passes through its corresponding ventricle, and thence 

 into the right ventricle, through the opening in the septum, which divides them (e). 

 The two currents of blood are here mixed, and re-propelled in this form , part to the 

 lungs, through the pulmonary artery (cj, and the remainder through the aortae (d.) 



The blood returned to the left ventricle, from the lungs, is passed through, or 

 thrown from, the left ventricle into the right, and hence, at once, by a dou- 

 ble passage through the pulmonary artery (e) and the aortae (f), to the lungs 

 and system generally. In the Crocodiles, and some species of Lizards, there is 

 but one ventricle ; and here the blood returning from the body and lungs is mixed 

 in the same way. In the various species of Frogs and Serpents, the heart is re- 

 duced to two cavities, a single auricle and ventricle ; and hence the blood which 

 has been fitted for circulation by the action of the air upon it in its passage 



