LUNGS. 



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550 



space inclosed by the boards and leather, but into a bladder contained 

 within that space. In this case, when the handle of the bellows is 

 raised so as to enlarge the cavity, the air will pass into the bladder, 

 and distend it so as to keep it everywhere in contact with the interior 

 of the cavity containing it. 



The acts of breathing are inspiration, by which air is drawn into 

 the lungs, and expiration, by which it is again expelled from them. 

 In inspiration the muscles that are attached to and form part of the 

 walls of the chest contract, and by raising the ribs and sternum, and 

 flattening the diaphragm, increase its capacity. The air within the 

 lungs (which are never empty even after the deepest expiration) is 

 thus for the instant rarefied ; but by the proportionally increased 

 pressure of the atmosphere upon the upper part of the larynx, a fresh 

 quantity of air immediately passes into the air-tubes, and maintains 

 the equilibrium of pressure between the air within and that without 

 the lungs. 



As soon as the action of the muscles of inspiration has ceased, 

 expiration commences; the lungs, distended in inspiration, contract 

 by their own elasticity, and expel a volume of air which in ordinary 

 circumstances is equal to that which they had just previously received. 

 As fart as they contract they are followed by the walls of the chest, 

 which ollapse partly by their elasticity, and partly by the pressure 

 of the atmosphere upon their exterior, which, when the lungs begin 

 to contract, is no longer exactly balanced by the pressure exerted 

 through the medium of the lungs upon their interior. The lungs 

 having thus contracted to a certain extent, the parts are restored to 

 the same condition as before inspiration, and in ordinary circumstances 

 that action is soon again commenced. 



The enlargement of the cavity of the chest in common inspiration 

 is thus effected : the diaphragm (figt. 3, 5) contracts ; its muscular 

 fibres, which are attached on the one hand to the interior of the lower 

 ribs, the tip of the sternum, and the front of the spine, and on the 

 other around a tendon (4, fy. 3) in its middle, shorten, and thus (as 

 the first set of attachments are fixed) they draw down the middle of 

 the muscle, lessen its convexity towards the chest, make it flatter, and 

 press its under surface upon the contents of the abdomen, so that the 

 abdominal walls become more prominent. At the same time, or just 

 previously, the intercostal muscles contract ; the two upper ribs, being 

 quite or nearly fixed at one end to the spine, and at the other to the 

 upper part of the sternum (fy. 2), serve as fixed points towards which 

 the upper intercostal muscles contracting draw the second ribs; these 

 being thus fixed, the second pair of muscles contract, and draw up 

 the third ribs ; and so on through the whole of the ribs, the lowest 

 serving, at the same time that they are drawn xipwards, for fixed 

 points, towards which the diaphragm, contracting all round its tendon, 

 may draw down its middle part and become flatter. 



The effect of the contraction of the intercostal muscles is not so 

 much to approximate the ribs (which would decrease the capacity of 

 the chest) as to force them further outwards and forwards, and thus 

 give the chest a greater width and depth at each part. Infiys. 2 and 4 

 it is seen that the ribs descend obliquely outwards and forwards from 

 the spine, and then ascend towards the sternum. They increase in 

 obliquity as they are taken from above downwards, and, except the 

 four last, they also increase in length in the same succession. The 

 length of the arc represented by each rib from the spine to the sternum 

 is fixed, for the substance of the ribs is bony or cartilaginous, and 

 almost unyielding ; when therefore one rib is fixed, and the intercostal 

 muscles between it and the one next below it contract, they must not 

 only draw the latter upwards, but must also turn it somewhat out- 

 wards, and raise the sternum, which is fixed' to its anterior extremity. 

 The direction of the rib becomes less oblique, but its length remaining 

 the same, the distance from the spine to the moveable sternum must 

 be increased at the same time with the distance from each rib to the 

 corresponding one on the opposite side. 



By these actions the cavity of the chest is increased in every direc- 

 tion ; in height by the descent of the diaphragm ; in width by the 

 turning outwards of the ribs ; in depth by the ascent of the sternum. 

 In quiet inspiration the greater part is effected by the diaphragm ; in 

 deep inspiration not only are all the muscles already mentioned con- 

 tracted, but a number of others capable of raising the ribs are called 

 into play, and the capacity of the chest is thus yet further increased 

 in the manner just described. 



In their medium state the lungs of a person of ordinary size and [ 

 in good health contain about twelve pints of air ; in perfectly easy I 

 breathing about a pint is drawn into them at each inspiration ; but j 

 from this the quantity may vary to as much as seven pints, according ! 

 to the force of inspiration, increased as it is, for example, when pre- 

 paring for a great muscular effort, or during singing, or before 

 coughing. 



f^uLt expiration does not need any muscular exertion; the elasticity 

 of the lungs, of the cartilages of the ribs, and- of the other parts dis- 

 tended in inspiration, is sufficient to restore them all to their previous 

 xtate. A limit is set to the collapse of the lung* by the unyielding 

 tissues of the walls of the chest. These cannot follow the contracting 

 lungs beyond a certain extent, and the elasticity of the lungs is not 

 sufficient for them to overcome the unbalanced pressure of the atmos- 

 phere upon their interior, which it would be necessary for them to , 

 do before they could contract from the interior of the walls of the | 



chest. If a wound be made into either pleural cavity, the lung at 

 once collapses completely, and expels nearly all the air it contained ; 

 for in this case the atmospheric pressure being admitted alike to the 

 exterior and the interior of the lung, its elasticity has but little to 

 overcome, and the air-cells and tubes immediately contract to the 

 smallest size of which they are capable. By the same means, when 

 both pleural cavities are opened at once death speedily follows, in 

 consequence of the collapse of both lungs and the suspension of all 

 breathing. 



The limit which the rigidity of the walls of the chest sets to the 

 elastic collapse of the lungs is never reached in ordinary respiration, 

 nor in extraordinary cases, except by the influence of other expiratory 

 powers besides those of the lungs. These powers are supplied chiefly 

 by the muscles of the abdomen, which contract with great force, and 

 through the medium of the contents of the abdomen force up the 

 diaphragm to an unusual height into the chest, at the same time that 

 certain muscles capable of depressing the ribs and sternum draw them 

 down and decrease the capacity of the chest in its depth and width. 

 Efforts of this kind are observable in coughing, sneezing, and all other 

 strong expiratory acts. 



For an account of the chemical changes accompanying these actions 

 see RESPIRATION. 



The development of the lungs has been recently investigated, and 

 the following is Kolliker's summary of what is known : 



" In the Mammalia the lungs appear a little after the liver, in the 

 form of two hollow protrusions of the anterior wall of the pharynx, 

 which are in close apposition, and soon become furnished with a 

 common peduncle the rudiment of the larynx and trachea and in 

 the composition of which the epithelial tube and the fibrous membrane 

 of the intestine take an equal share. In the further course of develop- 

 ment there springs from the extremities of the original protrusions a 

 continually-increasing number of arborescent processes, which differ 

 entirely in what may be observed in most other glands. From their 

 first formation they are always hollow, and in the sixth month the 

 air-cells are developed from their invariably clavate dilated extremities. 

 During this growth of the glandular elements the interior epithelium 

 extends itself by spontaneous multiplication of its cylindrical cells 

 (probably by division), whilst at the same time the fibrous layer sur- 

 rounding them also grows, and finally constitutes the fibrous membrane 

 of the bronchise and air-cells, together with the vessels and nerves. 

 In the second month, in the human embryo, the large pulmonary lobes 

 are already formed; and besides them smaller divisions also, - 16'" 

 in size, may be recognised, originating in the dilated extremities of the 

 bronchia?, which even at this time are considerably ramified. As 

 development proceeds, and the ramifications of the bronchia; are multi- 

 plied, these gland-granules, as they are termed, become more and more 

 numerous, and ultimately, in the fifth month, are aggregated so as to 

 form smaller lobules of 0'24"' (H8"' in size, each of which in all 

 probability is produced from a single gland-granule, or bronchial 

 termination, of the second month. Each of the gland-granules of 

 these lobules, which correspond with the secondary lobules of the 

 future lung, by continued budding, finally constitutes a primary lobule, 

 which, with air-cells of 0'025"' 0'03'" in size, first becomes distinctly 

 visible in the sixth month, although up to the time of birth uew 

 alveoli are constantly superadded. In the new-born child the secondary 

 lobules measure 2'" 3'" 4'" ; the alveoli, before they are filled with 

 air, 0-03'", and after the first inspiration, 0'03'" 0'04'" 0'06"' ; the 

 latter at this time appear to exist in the same number as in the adult, 

 the further increase of the lungs proceeding only from the expansion 

 of all its parts. 



" TLe investigation of the lungs," continue the translators of 

 Kiilliker, "presents no real difficulty, except in one point; that is, 

 with respect to the relation of the pulmonary cells to the terminations 

 of the bronchia! ; but here the difficulties are very considerable. In 

 recent preparations it is obvious that the air-cells communicate in 

 many ways, and in any case that they are not merely terminal on the 

 extremities of the bronchise. If it be desired to investigate the whole 

 subject, inflated and dried lungs (it is better in an inflated lung to tie 

 off an end and dry it by itself), or corroded preparations, or lungs 

 injected with uncoloured substances (wax and resin), are most suitable ; 

 and with such a definite result will be obtained, after a series of 

 observations. Before the injection of the bronchia) is proceeded with 

 the air must be exhausted in the air-pump, for which purpose also, 

 though less conveniently, a well-fitted syringe may be employed. Tho 

 injection of the bloodvessels is readily effected, and the preparation 

 should be kept wet ; sometimes when injected with opaque material, 

 sometimes following the processes of Schroder and Harting, with 

 transparent substances (Prussian blue, &c), dried preparations are to 

 be preferred. The air-cells and bronchia:, the larynx and trachea, are 

 readily examined. The epithelium of the air-cells is obtained in largo 

 quantities in every section through the lung, as well as ciliated cells. 

 If it be wished to study the alveoli, the air must previously be care- 

 fully removed. These are best displayed in man, in whom also all 

 other parts, such as cartilage, elastic elements, muscles, and glands, 

 are easily obtainable." 



(Kolliker, Manual of Human Ilistoloyy, translated for the Sydenham 

 Society by Busk and Huxley.) 



LUNG-WORT. 



