210 



To lighten birds sJill farther, they have large mem- 

 branes extending to the bottom of their bellies, into 

 which the air is received, where, by the h^at of the 

 body it is expanded into twice or thrice the dimensions 

 of the external air; and this they can either compress 

 by the muscles of the abdomen y or expire more or iess r 

 in order to their descending swifter or slower, in what 

 degree they please. 



3. As to the brain of birds, whereas in men tine 

 cortical part of the brain is outermost, in birds it is in. 

 ncrmost, and the membrane that covers the upper ven- 

 tricle is medullary. The ventricles likewise uic bituut- 

 cd above, near the upper part of the skull. 



Perhaps before we proceed, it may be well to pre- 

 mise one general observation, that the structure of 

 birds is in many respects different from those both of 

 men and beasts, having several parts which these want t 

 and wanting others which they have. Besides, there 

 are great variations in the contrivance of parts which 

 are common to both, all wisely adapted to their dif* 

 ferent conditions and manner of life* 



Parts peculiar to birds are, 1. The bill. 2. The 

 membrane to draw over the eyes. 3. Feathers and 

 wings. The parts wanting in birds are, 1, Teeth and 

 lips. 2. The bladder, which they do not need, as 

 they drink no more than just to moisten their food. 

 Variations in the parts of birds from those of men and 

 beasts are, 1. In the ear, which is of a very peculiar 

 make. 2. In the division of the aorta. 3. In the 

 spinal marrow, which is divided into two in the middle 

 of the back. 4. Tn the bones, which are all hollow. 

 5. In the heart, which has a fleshy valve at the month 

 of the vena cava. 6. In the lungs, which are strong' 

 ly joined to the back, for the greater conveniency of 

 ilight. 7. In the stomach ; birds have two or more, 

 to supply the want of ^chewin,g,-~8. lii the legs and 



