THE ENGLISH SPARROW 385 



is into this region that the principal digestive fluids (those from 

 the hver and pancreas) are poured. The great middle portion 

 of the intestine, together with a pair of blind sacs (caeca) 

 opening into it, serves for the absorption of the digested sub- 

 stances and, finally, the indigestible remnants are collected 

 in the rectum, whence they pass, through a cloaca, to the 

 external vent. The cloaca receives also the excretory products. 



Organs of Respiration. — These also reach an extraordinary 

 complication in birds. From the larynx, which, as in man, is 

 supported by cartilages but is not, in birds, the vocal organ, 

 a long windpipe passes to the two lungs. Where the wind- 

 pipe begins to sul^divide to form the bronchi of the lungs, there 

 is found the vocal organ of birds — the syrinx (Fig. 35.5) ; 

 nothing like it is found in any other class of vertebrates, and 

 it gains a very complex structure in the singing-birds. As it 

 would take a long time to describe the mechanism of the 

 syrinx, this will not be attempted; suffice it to saj^ that the 

 sound is produced by the vibration of a membrane which is 

 stretched part way across the windpipe. 



The respiratory organs proper are of two kinds — the lungs 

 and the air-sacs. The lungs (Fig. 355) are relatively small 

 organs and are not mere bags, as in the lower orders of verte- 

 brates, but spongy masses rich in blood-vessels. Through this 

 sponge-work the pure inspired air rushes on its way to the air- 

 sacs which he beyond. Consequently the blood of the lungs 

 is bathed by a current of fresh air, and this insures that it shall 

 receive a maximum of oxygen and this, in turn, results in the 

 bird maintaining a high body temperature, 103° F. (40° C), in- 

 stead of 98° F. (37° C), as in mammals. The air-sacs of birds 

 correspond, in a way, with the little terminal sacs of the bron- 

 chi in ourselves, but some of these have become mimensely 

 2c 



