CHAMBERS'S INFORMATION FOR THE PEOPLE. 



mucous membrane, in which is imbedded a net- 

 work of capillaries, through which all the blood 

 is in turn driven by the heart. The moist par- 

 tition between the blood in this network and 

 the air in the interior of the lungs, is so thin as to 

 allow an interchange between the gases of the 

 blood and the gases of the air ; that is to say, 

 oxygen passes from the air in the air-cells into 

 the blood, while carbonic acid gas and aqueous 

 vapour pass outwards from the lungs into the air 

 in the air-cells. This is a purely physical phenom- 

 enon, dependent on the laws of diffusion and 

 admixture of gases through animal membranes. 



Fig. 4. 



In the higher animals and in man, these essential 

 parts are much complicated and modified in a 

 variety of ways. The anatomical details may be 

 considered under the following heads. Firstly, the 

 lungs must afford by their internal arrangement 

 an immense extent of internal mucous membrane, 

 covered by vascular network, through which, as 

 in the simpler form, the blood flows in innumerable 

 minute streamlets, only separated by an extremely 

 thin membrane from the atmospheric air that 

 has been inhaled ; secondly, there must be such an 

 arrangement of the circulating system, that fresh 

 blood may be perpetually driven from the right 

 side of the heart through the lungs, and onward 

 to the left side of the heart ; and thirdly, there 

 must be arrangements for the frequent and regular 

 change of the air contained in the lungs. 



We shall first consider the lungs and the pass- 

 ages leading to them. The back of the mouth 

 or pharynx is connected with the outer air in two 

 ways namely, by the nasal passages and nostrils, 

 and by the mouth. Through either of these 

 channels, the air may pass to and from the 

 lungs, but the nostrils are, properly speaking, 

 the entrances to the respiratory system. Behind 

 the root of the tongue, we find a chink or 

 aperture, the glottis, bounded laterally by two 

 folds of membrane called the vocal chords, 

 which may be more or less widely separated from 

 each other. This chink or aperture is guarded 

 by a leaf-like lid, the epiglottis, which can be 

 closed when expedient, so as to prevent the 

 120 



entrance of particles of food, drink, &c. The 

 glottis opens downwards, into a box-like chamber 

 called the larynx (which is the organ of voice), 

 and leading downwards from the larynx runs the 

 trachea, or windpipe, which is kept permanently 

 open for the passage of air, by cartilaginous rings, 

 that surround the anterior two-thirds of it. These 

 are united, and the back of the tube is formed by 

 a fibrous membrane or muscle. The windpipe, 

 which is easily felt by the hand, and lies just 

 below the projecting part of the larynx, popularly 

 known as Adam's Apple, is about four and a 

 half inches in length, and about three-fourths of 

 an inch wide. Passing into the cavity of the 

 chest, it divides into two branches, which are 

 termed the right and left bronchi. Each bronchus 

 enters the lung of its own side, and divides 

 into a great number of smaller tubes, called 

 the bronchial tubes, which again go on subdivid- 

 ing. These finest tubes end in elongated dilata- 

 tions, averaging ^th of an inch in diameter, 

 which are called the air-cells. If we can con- 

 ceive a bunch of grapes with its stem and all 

 its minute branches, and the grapes attached 

 to the ends of them to be hollow, we get a good 

 idea of the mode in which the lung is con- 

 structed, except that it does not represent all the 

 sacculation or partitioning of the terminal cells. 

 It is in consequence of the air included in these 

 cells that the lungs have their soft spongy feeling, 

 and crackle when compressed between the fingers. 

 Each lung is invested by its own investing serous 

 membrane, termed the pleura, which serves the 

 double purpose of facilitating the movements 

 necessary in the act of respiration, and in sus- 

 pending each lung in its proper position. 



The blood is being perpetually changed and 

 driven in a constant current through the lungs 

 by the action of the heart, the venous or impure 

 blood being collected in the right ventricle, and 

 thence conveyed by the pulmonary artery into 

 the lungs. In these, again, it gives off carbonic 

 acid and aqueous vapour, and absorbs oxygen (as 

 already described) ; and after these changes, it is 

 collected, and returned to the left auricle by four 

 vessels called the pulmonary veins. 



The mode in which the air is renewed in the 

 lungs next requires notice. This is effected by 

 the respiratory movements, which consist in alter- 

 nate acts of inspiration and expiration, with an 

 intervening pause before the process is renewed. 

 An adult man in a sitting position performs the 

 respiratory act from thirteen to fifteen times in 

 the minute, but much more rapidly if taking 

 exercise. At each inspiration, about 30 cubic 

 inches of air are inspired, and at each expiration 

 nearly the same volume is exhaled, difference 

 being allowed for temperature, which in the ex- 

 haled air may equal that of the blood. From 

 300 to 400 cubic feet of air thus pass through 

 the lungs of a man at rest in the course of 

 twenty-four hours, and these are charged with 

 carbonic acid, and deprived of oxygen to the 

 extent of nearly 5 per cent. ; or, to put it in 

 another form, about 18 cubic feet of the one 

 gas are taken in, and of the other gas are given 

 off. The quantity of carbon thus excreted in the 

 form of carbonic acid gas is nearly represented 

 by eight ounces of pure charcoal The amount 

 of watery vapour separated by the lungs varies 

 from six to twenty ounces daily, according to the 





