RESPIRATION 



of them, and that their total surface is almut 2000 

 square feet. The walls of the air-chambers are 

 formed of n tliin membrane in which tin- blood ; 

 and Ivinpli capillaries ramify. Minute openings 

 lead from tin* iiir chaiuliers into the lymph spaces 

 of tin- membrane. The luemhranoiiH wall- are 



partly formed of 

 elastic tissue. It 

 is this that give-, 

 to the lungs their 

 rl.-i.-tir.ty. The 

 larger air passages 

 (trachea and 

 bronchi) are kept 

 otien by horseshoe- 

 UUMtt plates of 

 cartilage ; muscles 

 stretch between 

 the poles of the 

 horseshoe, com- 

 plete the ring, and 

 permit the sixe of 

 tin- passages to 

 vary, at the same 

 time resisting 

 over-distension 

 when the internal 

 pie- -n re rises. 

 These larger air- 

 passages are lined 

 by a mucous mem- 

 brane, containing 

 mucous glands ; 

 the innermost 



The Trachea (windpipe), Bronchi, layer is a ciliated 

 and one of the Lungs in section, epithelium; the 



cilia lash upwards, 



and thus keep the passages free from mucus 

 and remove foreign particles. As the passages 

 become smaller they lose their cartilages, and the 

 muscles form a continuous circular layer. The 

 lungs are invested by a membrane ( the visceral 

 pleura). At the root of the lungs this membrane is 

 continuous with a membrane which lines the chest- 

 cavity (the pai i.-t al pleura). The space between 

 the two is the plenral cavity ; it is in reality a large 

 lymph space, and communicates with the lymphatic* 

 of tlie pleura. Owing to the air-pressure within 

 the lungs the two pleura- are closely pressed to- 

 gether, the lungs entirely filling the chest cavity. If 

 the chest-wall lie punctured the lungs partially 

 collapse owing t their elasticity, and the respira- 

 tory movements are unable to move the air in 

 the lungs. 



The r/i, it is an air-tight chamber enclosing the 

 lungs and the heart. The walls of the chest are 

 formed of bone- (the ribs, -termini, and hacklxmc) 

 ami muscles; the boon AM muscles are so arranged 

 that the size of the chest-cavity can be altered. In 

 this way the chest acts as a bellows and moves air 

 in and out of the lung-. The ribs are slo|x*d slightly 

 downwards, especially after an expiration : when 

 an inspiration is taken certain muscles fix the 

 up|H;r ril, and those muscles connecting the ribs 

 to each other contract and the ribs are raised, and 

 thus the size of the chest-cavity is increased. At 

 the same time a flat muscle called the Diaphragm 

 (q.v.), which separates the chest-cavity from the 

 rest of the body-cavity, and which after an expira- 

 tion is arched upwards (by the pressure of the 

 abdominal viscera upon it, tin- \i-ccra in turn being 

 pressed U]>on by the abdominal walls), forcibly 

 contract*, becomes Matter, and therefore enlarges 

 the size of the chest-cavity, forcing the abdominal 

 viwera downward* and causing the abdomen to 

 protrude. (The relation of the lungs to the other 

 main organs will lx> seen in the illustration at 

 ; -,-e also that at DIAPHRAGM.) In 



these two ways, then, the size of the chest cavity 

 may be increased. The result of this enhugcmeiit 

 is that the | ne me of the air within the ea\ ilics nf 

 tin- lungs is lowered; air therefore from without 

 iii-lie- through the nostrils (one ought not to 

 breathe through one's mouth) down the windpipe 

 into the lungs, and thus a fresh supply of oxygen 

 is introduced. The movements which produce this 

 result are known as the inspiratory movement*. 

 In making an cxpiiatinn the reverse effects are 

 produced ; the chest-cavity is made smaller, the 

 pi. me of the air in the lungs increases, and some 

 rushes out through the nostrils into the air until 

 the pressures inside and outside are equalised. An 

 ordinary expiration is effected by the elasticity of 

 the lungs, by the fall of the ribs, UMpportM by 

 the contraction of the muscles that caused an 

 inspiratory movement, by the elasticity of tin- 

 cartilages of the rilw which were twisted during 

 inspiration, and by the elasticity of the alidoniinal 

 wall which was forced outward's by those viscera 

 pushed downwards by the diaphragm. An ordinal \ 

 inspiration is therefore the result of a number t 

 active muscular contractions, while an ordinary 

 expiration is the result of mere passive elasticity 

 of the parta concerned. There are certain other 

 respiratory movements to be considered. During 

 inspiration and expiration the glottis (the opening 

 between the vocal chords of the larynx; see the 

 illustration at LARYNX) undergoes a rhythmical 

 widening and narrowing ; this movement is {greater 

 in forced than in quietbreathing. And during in- 

 spiration the nostrils dilate; in most cases perhaps 

 the inspiration has to be rather a forced one !- 

 fore they do so. Forced respiration occurs when 

 the supply of oxygen is insullicient, or when 

 carlMinic acid accumulates in the blood. Any 

 muscle that can aid in enlarging and decreasing 

 the size of the chest-cavity is called into play. The 

 average amount of air, in the case of an individual 

 5 feet 8 inches in height, that goes in and out of the 

 lungs at each in-piiation and expiration is about 

 20 cubic inches; this is called the tidal air. Ity 

 menus of forced inspirator}- movements the ingoing 

 tide may lw increased by 120 cubic inches; \y 

 means of a forced expiration the outgoing tidal air 

 may be increased by 90 cubic inches. After the 

 most forced expiration possible there always remain 

 within the lungs about 90 cubic inches of air. So 

 that if we take as deep a breath ax possible, and 

 then make as forced an expiration as we can, we 

 shall drive out 120 + 20 + 90 = 230 cubic inches of 

 air. This is termed the respiratory capacity. Since 

 the tidal air is only 20 cubic inches, and 180 cubic, 

 inches remain in the chest after an ordinary cxpiui 

 tion, it follows the air directly changed during 

 respiration is not that really within the lungs 

 themselves, but is that within the nose, windpipe, 

 and larger bronchi, the pipes that result from the 

 branching of the windpipe. Therefore the changes 

 of the nir within the essential parts of the lungs 

 are the result of diffusion lietween it and the purer 

 air of the bronchi, aided by the rush with which 

 the tidal air Hows in. 



The ordinary respiratory movements differ in tin- 

 two sexes and at different periods of life. In young 

 children the chest is altered in size chiefly by the 

 movements of the diaphragm, and the protrusion of 

 the abdominal wall during inspiration is therefore 

 very marked. In men also it is the diaphragm 

 which is chiefly operative, but the ribs are also 

 moved. In woiui-n it is the movement of the ribs, 

 especially the upper ones, which is the most exten 

 sive. Tlie respiratory rhythm is the relation of the 

 acts of inspiration and expiration to each other as 

 regards time. It may lie expressed as follows : 

 In. = 3, Ex. = 4, pause = 3. The numbei of 

 respirations in a healthy person is almut fourteen 



