382 



THE HUMAN BODY. 



small amount of respiration is carried on in the alimentary 



canal, the air swallowed with food 

 or saliva undergoing gaseous ex- 

 changes with the blood in the gas- 

 tric and intestinal mucous mem- 

 branes. The amount of oxygen 

 thus obtained by the blood is 

 however very trivial, as is that 

 absorbed through the skin, cov- 

 ered as it is by its dry horny non- 

 vascular epidermis. All the really 

 essential gaseous interchanges be- 

 tween the Body and the atmos- 

 phere take place in the lungs, two 

 large sacs (lu, Fig. 1) lying in the 

 thoracic cavity, one on each side 

 of the heart. To these sacs the 

 air is conveyed through a series of 

 passages. Entering the pharynx 

 through the nostrils or mouth, 

 it passes out of this by the open- 

 ing leading into the larynx, or 

 voice-box (a, Fig. 122), lying in 

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

 is seen in Fig. 107) ;' from the larynx passes back the trachea 

 or windpipe, b, 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 bronchial tubes, 

 within the lung on its own side; and the smallest bronchial 

 tubes end in sacculated dilatations, the infundibula of the 

 lungs, the sacculations (Fig. 124) being the alveoli; the word 

 " cell " being here used in its prim- 

 itive sense of a small cavity, and 

 not in its later technical significa- 

 tion of a morphological unit of 

 the Body. On the walls of the 

 air-cells the pulmonary capillaries 

 ramify, and it is in them that the 

 interchanges of thje external res- 

 piration take place. 



Structure of the Trachea and 

 Bronchi. The windpipe may 

 readily be felt in the middle line of the neck, a little below 



Vis. 122. The lungs and air- 

 passnges seen from the front. On 

 the left of the figure the pulmo- 

 nary tissue lias been dissected 

 away to show the ramifications of 

 the bronchial tubes, a, larynx ; 

 6, trachea ; d, right bronchus. 

 The left bronchus is seen entering 

 the root of its lung. 



FIG. 123. A small bronchial tube, 

 a, dividing into its terminal branch- 

 es, c ; these have pouched or saccu- 

 lated walls and end in the saccu- 

 lated infundibula, b. 



