130 



HIGH SCHOOL ZOOLOGY. 



cceca,, which open into the anterior end of the hirge intestine. 



CS> 10. Certainly the most 

 remarkable feature about 

 tlie respii-atory system of 

 birds is the develojiment 

 of air-sacs, which i-eceive 

 their air from the lungs, 

 and are situated partly 

 among the viscei'a of the 

 body-cavity, partly be- 

 tween tlie muscles, and 

 underneath the skin of 

 I/he body, and finally 

 within the bones, dis- 

 placing the marrow in 

 these. The function of 

 these air-sacs is not con- 

 fined to supplementing 

 the size of the lungs, 

 (although a certain inter- 

 change of gases between 

 the blood and the con- 

 tents of the air-sacs must 

 take place), but they like- 

 wise serve to render the 

 body specifically lighter 



Fiy. 94— Viscera of the FowL (after Brandt). 



oe, iBsophagns; ig, crop; tr, trachea; m, muscle; 

 Ui, syrinx ; p, lung^; c, heart; h, liver; dh, hepatic 

 ihict ; vf, gall-bladder; dch, bile-duct; iiv,]>roveii- 

 triculus; sp, spleen ; v, ;j;i/',zard ; d, duodenum ; 

 lia, pancreas; i, small intestine.; toe, its ca-ca ; 

 fo, eiTiJ-follicle burst ; o, eiigs ; od, os iduct part- 

 ly slit open containini^ a mature egg, o' ; cl, clo- 

 aca ; Bz, Bursa Fabrieii. 



(especially as the bodily temperature is high), and thus more 

 adapted foi' flight. As the bird's locomotion involves much 

 muscular exertion, both the respiratory and circulatory systems 

 ai-e more perfect than in tlie reptiles, the presence of four 

 chambers in the heart, allowing the complete separation of the 

 blood which has been returned for aeration, from that which is 

 sent out by the heart to the system. 



11. In the lower Vertebrates the voice is little developed, 



