RESPIRATION IN REPTILES AND BIRDS. 373 



on in these animals ( 561), the interruption to the flow of blood 

 through the lungs does not, as in the higher classes, produce a stagna- 

 tion of the general current through the body ; and the blood receives, 

 in its course through the skin, a sufficient amount of aeration for the 

 support of life. Indeed, at a low temperature, the influence of water 

 on the skin is sufficient (by means of the air included in the liquid) to 

 remove the small amount of carbonic acid then ready for excretion, and 

 to supply the requisite amount of oxygen ; and Frogs may thus live 

 beneath the water for any length of time, without coming to the surface 

 to breathe. But with the rise of the temperature of their bodies, their 

 blood requires a higher degree of aeration ; and they then come to the 

 surface to take in air by the mouth, which aerates the blood through 

 the lungs. It appears that, during the heat of summer, the pulmonary 

 respiration, and the influence of the water on the skin, are not sufficient ; 

 as it is found that Frogs die, if they are confined to the water under 

 such circumstances, their natural habit being to quit the water at 

 such times, so that the air may exert its full influence on their skin as 

 well as on their lungs. They do not, however, quit the neighbourhood 

 of water, and soon die if exposed to a dry atmosphere, for, if the skin 

 become dry, its aerating function can be no longer performed. The 

 same result happens if the passage of gases through the skin be impeded 

 by smearing it over with any unctuous substance. We shall presently 

 find reason to believe that this cutaneous respiration is a very important 

 part of the function, even in Man and Mammalia. 



672. The class of Birds presents a most striking contrast to that of 

 Reptiles, in regard to the energy of the respiratory function, and the 

 extent of the apparatus destined to its performance. The air-cells are 

 considerably diminished in size, so that the extent of surface over which 

 they expose the blood to the air is greatly increased ; and there even 

 seems reason to believe that the air comes into direct contact with the 

 vessels of the very close capillary plexus which intervenes between the 

 air-cells. But the subdivision of the lungs is not carried to the same 

 degree of minuteness as it is in Mammalia ; and the required extent of 

 surface would not be afforded by the lungs alone. In addition to these 

 organs, we find large air-sacs, communicating with them, disposed in 

 different parts of the body, such as the abdominal cavity, the inter- 

 spaces among the muscles, the spaces between the muscles and the 

 skin, &c. These very greatly increase the respiratory surface, their 

 lining membrane being extremely vascular, and adapted to expose the 

 blood to the influence of the air. In most Birds, the bones themselves 

 are hollow, and the lining membrane of their cavities serves as an addi- 

 tional aerating surface, the air being introduced into the interior of the 

 bones, by canals that communicate directly with the lungs. So free is 

 this communication, that the respiration has been known to be main- 

 tained through the fractured humerus of an Albatross, when an attempt 

 was made to destroy the bird by compressing its trachea. Thus the 

 respiratory surface is extended into the remoter parts of the system, 

 very much as in Insects ; and the hollowness of the bones, together 

 with the presence of numerous air-sacs in different parts of the body, 

 contribute to diminish its specific gravity. The large quantity of air 



