N0V..1893. THE AIR-SACS OF BIRDS. 347 



questioned that they exist partly for purposes of respiration. The lungs 

 themselves are small and inelastic, and the diaphragm, whether it 

 represent that of mammals or not, certainly does not do the same work. 



By the action of muscles the body-cavity is expanded, a vacuum 

 is created in the sacs, and the air rushes into them through the lungs. 

 But it is difficult to believe that the air-sacs fulfil no purpose beyond 

 that of respiration. If that is the case, they are far larger than 

 necessary. 



I have measured the cubic content of the lungs of a pigeon as 

 nearly as I could, and found it to be ^ inch. The aggregate length 

 of the air-sacs was about 3f in., the depth about i in., the breadth 

 about f in. This gives a cubic content of i^|. According to this 

 estimate, therefore, the air-sacs can hold more than five times the 

 amount of air the lungs would hold, even if they were mere bags. 

 And probably this estimate is decidedly below the mark. This dis- 

 proportionate amount of air is of some use in breathing. When the 

 bird exhales, comparatively fresh air from the sacs is driven into the 

 lungs, so that exhalation and inhalation alike renew the supply of 

 oxygen ; but even when this is allowed for, the air-sacs are far more 

 spacious than is necessary for breathing alone. We have to consider, 

 therefore, whether they do not fulfil some useful purpose in addition 

 to their respiratory functions, and if we reflect upon the subject, it is 

 difficult to avoid coming to the conclusion that a bird's temperature is 

 regulated mainly by the lungs and their extensions. This will seem 

 more than probable if we consider the means by which the tempera- 

 ture of the human body, when in health, is kept almost at the same 

 point, however much that of the surrounding air may vary. Heat is 

 lost (i) through skin by conduction, radiation, and evaporation ; (2) by 

 respiration ; (3) to a small extent through the excreta. Besides this 

 the temperature of the body is partly controlled by the vaso-motor 

 nerves which regulate the flow of blood to particular parts. And, that 

 this is not the only way in which the nervous system governs the 

 temperature, there is indirect evidence in the fact that when a warm- 

 blooded animal is subjected to urari poisoning it behaves like a cold- 

 blooded animal. It has no longer any power of generating heat 

 within itself in order to withstand external cold, or, when exposed to 

 heat, of keeping itself cooler than the surrounding atmosphere. 



Whatever the exact nature of this nervous apparatus may be, it is 

 certain that in man the skin plays a very important part in the 

 regulation of temperature, and that it is mainly by evaporation that 

 it does its work.' Radiation is constant, but is much checked by 



1 Dr. Michael Foster ("Text-book of Physiology," p. 464, 1883 edition) writes: 

 ' ' It has been calculated that the relative amounts of the losses by these several channels 

 are as follows : in warming the urine and faeces about 3, or according to others 6 

 per cent. By respiration about 20, or, according to others about 9 only per cent., 

 leaving 77, or alternatively 85 per cent., for conduction and radiation and evaporation 

 from the skin." 



