354 



THE HUMAN BODY. 



without the intervention of the blood. But in tiie great 

 majority of air-breathing animals the arrangement is dif- 

 ferent; the air-tubes leading from the exterior of the body 

 do not subdivide into branches which ramify all through it, 

 but open into one or more large sacs to which the venous 

 blood is brought, and in whose walls it flows through 

 a close capillary network. Such respiratory sacs are called 

 lungs, and it is a highly developed form of them which is 

 employed in the Human Body. 



The Air-Passages and Lungs. In our own Bodies some- 

 small amount of respiration is carried on in the alimentary 



canal, the air swallowed with 

 food or saliva undergoing gas- 

 eous exchanges with the blood 

 in the gastric and intestinal mu- 

 cous membranes. The amount, 

 of oxygen thus obtained by the 

 blood is however very trivial, a& 

 is that absorbed through the- 

 skin, covered as it is by its dry 

 horny non-vascular epidermis. 

 All the really essential gaseous 

 interchanges between the Body 

 and the atmosphere take place- 

 in the lungs, two large sacs (In? 

 Fig. 1) lying in the thoracic 

 cavity, one on each side of the 

 heart. To these sacs- the air is 



FIG. 104. The lungs and air-pas- Conveyed 



through 



a series of 



sn.iivs seentrom tlie tront. On tne - r > OCC!0n . r v C , T7n4oiM-nrr 4-1-m -rL o v-f-n tr 

 left of the figure the pulmonary passages. JintCling til 6 pliai\nx 



the * 08trils or mouth, 

 & P asses Ollt Of tllis b J the 



chus is seen entering the root of its i n g leading into the larynx, or 



voice-box (a, Fig. 104), lying in 



the upper part of the neck (the communication of the two 

 is seen in Fig. 89); from the larynx passes back the 

 trachea or windpipe, #, which, after entering the chest cavity, 

 divides into the right and left bronchi, d, e. Each bronchus 

 divides up into smaller and smaller branches, called Iron- 



