ORGANS OF VOICE AND RESPIRATION 119 



halves. 4th, A cell for the heart. 5th, A cell situated in front of 

 the breast for the bronchi, which it surrounds, together with the 

 inferior larynx. A still greater extension of the air-cells occurs in 

 some Birds. Thus, the Roller (Coracias) is provided with a pair 

 of large air-cells beneath the skin of the head and neck, and .these 

 communicate with the nasal cavity, but not with the trachea. The 

 distribution of air throughout the body is nowhere more extensive 

 than in the Booby (Sula) and the Pelican. In these genera the 

 lateral cells of the trunk are uncommonly large, and separated by 

 two partitions into three large chambers, from the most anterior of 

 which the air gets beneath the axilla under the integument, and 

 fills the space upon the breast and belly from the furcular to the 

 pubic bones. Several larger and various smaller cells are also met 

 with, and the fat, which is generally abundant in such situations, 

 is here wanting. The air-cell above the great pectoral muscle and 

 on the inferior part of the neck is particularly large, and the deli- 

 cate cellular tissue here forms partitions including cells several lines 

 in diameter, which are continued almost beneath the epidermic layer 

 of the skin as far as the quills of the contour-feathers, but not into 

 their interior. These cellular air spaces are further distributed 

 beneath the short investing feathers of the wing, and between the 

 quills of the great primaries. Upon the middle and upper part of 

 the body tegumentary air-cells do not exist ; and upon the head 

 there is found beneath the crisp-feathers covering the occipital re- 

 gion, only a single solitary cell partitioned off into some small spaces. 

 The communication which exists between the pneumatic cells of 

 Birds and the interior of many of their bones, the latter being for 

 this purpose devoid of medullary tissue, and thus rendered permea- 

 ble to air, has been already mentioned' in treating of the skeleton. 

 The Apteryx offers a striking contrast to the Pelican in being en- 

 tirely devoid of air-cells, and is hitherto the only known exception 

 of this kind among Birds. This extensive distribution of the at- 

 mospheric air throughout the body of Birds contributes obviously, 

 by highly oxygenating the blood, to increase the general activity of 

 their arterial system, conditions which are manifested in the greater 

 number of its pulsations within a given time, and its more elevated tem- 

 perature (107 to 1 10 Fahr.), as contrasted with that of the Mammalia. 

 The mechanism of the function of respiration, according to the 

 recent special researches of Dr. Edward Weber, is performed in the 

 following manner. The ramifications of the bronchi with the net- 

 work of tubes they form within the lung are during inspiration 



