50 VERTEBRATA : MAMMALIA. 



through the body; and it is called double because the 

 blood passes through two sets of capillary vessels one set 

 belonging to the lungs, and the other set to the body. 



The circulation of tlie blood through the lungs is called the pul- 

 monary circulation, and that through the body, the systemic circulation. 



The respiratory apparatus of Mammals consists mainly 

 of lungs, and an air-tube or trachea, to carry the air to and 

 from these organs. 



o 



At no stage of their existence are their " visceral arches " 

 furnished with branchial or gill-like appendages.* 



FIG. 58. 



fe 



J.arynx. 



Trachea. 



Bronchi 



^J Left lung. 



Bronchial ramusculse. ... S, 



tfi 



Lungs and Trachea of the highest Mammal Man. 



* In the embryonic condition of all the Vertebrata, the part of the body- 

 wall which lies at the sides, and just behind the mouth, exhibits a series of 

 vertical parallel thickenings; these thickenings are called the " visceral 

 arches." The spaces between these arches become thinner and thinner, 

 and at length clefts or ' ' branchial fissures " take the place of the thin spaces, 

 and thus there are openings from the exterior surf ace of the animal into the 

 anterior portion of the digestive cavity. In the higher adult vertebrates 

 the " branchial fissures " are obliterated, or so modified as apparently to dis- 

 appear; but in Fishes and certain Batrachians they remain in connection 

 with the gill-bearing arches. 



