HUMAN PHYSIOLOGY. 



in immbT. tho ritfht and the left, and occupy the corresponding 

 ;.-ti-r.il .M\ :ti.- of the cheat ; they are conical in nhape ( the 

 .I,. I MI' tin) cone being placed tho highest, and extending 

 into tin- rot of the nock from an inch to an inch and a half 

 above the level of the fir at rib ; the broad base of ooch lung 

 .!!! tho diaphragm, and extends lower behind than in 

 Each lung is composed of two parts, called tho upper 

 .ind tho lower lobes, which are separated from each other by a 

 In tho right lung the upper lobe is partly split into 

 two by a shorter fissure, so that the right lung is said to have 

 tin-.-.- lobes, whilst the left has only two. The right long is 

 always the largest ; it is broader than the left, in consequence of 

 the greater divergence of the heart to the left side ; but this is 

 in some measure compensated for by its being shorter, because 

 of tho liver forcing up tho diaphragm to a higher level on the 

 right side. About the centre of the inner surface of each lung 

 is a spot where the bronchus, the pulmonary artery, and pul- 

 monary vein, and nerves enter the substance of the lung ; these 

 structures, together with arteries and veins proper to the 

 bronchus and the bronchial glands, are all enclosed in a process 

 of tho pleura, and form what is called the root of the lung. 

 Tho two lungs taken together in the adult weigh from two 

 pounds and three-quarters to three pounds ; they are heavier in 

 the male than the female. The colour of the lung varies with 

 the age of the individual. At birth they are of a pinkish white; 

 in the adult they become mottled with patches of a dork slate- 

 colour, in consequence of the deposit of colouring matter of a 

 carbonaceous character ; as old age advances, these patches 

 become nearly black. The substance of the healthy lung is 

 light and spongy, floating in water, and crackling when handled, 

 a frothy fluid being squeezed out. In disease it often becomes 

 solid, and is then heavier than water, and contains no air ; this 

 is one of the results of inflammation of the lungs. 



We must now consider the minute structure of these curious 

 organs. The substance proper of the lung is enclosed in a 

 serous coat, derived from the pleura, and is made up of an 

 infinite number of small divisions, called lobules, which, though 

 closely bound together by connective tissue, are still quite distinct 

 from each other. Each lobule is composed of a number of cells, 

 called air-cells, clustered upon, and opening into, the tormina] 

 branches of the bronchi, with the minute divisions of the blood- 

 vessels and nerves. When the bronchus enters the lung it 

 divides into two, and these branches repeat the process until 

 the ultimate ones have a diameter of less than the ^ of an inch. 

 In the largest branches the structure remains the same as in 

 the bronchus ; they have walls, formed of tough membrane and 

 imperfect cartilaginous rings, by which they are held open ; 

 but as they attain their greater degree of minuteness, the walls 

 consist simply of membrane. Into these smaller ones the air- 

 cells open, and over them the pulmonary capillaries spread their 

 close network. The air-cells vary much in form, according to 

 the amount of pressure to which they are subjected ; their walls, 

 which are nearly in contact, are formed of very thin membrane 

 The size of an air-cell is from the 5^5 to the ^ of an inch in 

 diameter ; they communicate freely with each other, and are 

 as before stated, arranged in groups along the sides of the 

 bronchial tubes. It has been estimated that the total number 

 in the lungs exceeds 600,000,000. Outside of these cells anc 

 tubes the capillary plexus is so dense, that the meshes are 

 narrower than the vessels which compose them ; the capillaries 

 here have an average diameter of the ^^ of on inch. Thus 

 the blood is brought into the most intimate relation with the 

 air contained in these myriads of cells, there being nothing 

 interposed between them but the very thin walls of the cells 

 and capillaries, and frequently, this bringing the blood and al- 

 together, is even more perfectly provided for, as one capillary 

 will often have a layer of air-cells on each side of it. The cells 

 of one lobule do not communicate with those of another, anc 

 consequently if the bronchial tube going to a lobule become 

 stopped, the supply of air to that lobule ceases, and it is ren 

 dered useless. 



The function of respiration consists of two distinct acts 

 called respectively inspiration (by which the lungs are inflatec 

 with air) and expiration (by which the air, after having servec 

 its purpose, is driven out of the lungs). To understand this 

 process, we must fix firmly in our minds the conditions under 

 which it is performed. The highly elastic lungs are enclosed in 

 the cavity of the thorax, the bony framework of which is com 



pleted in all iU deficient part* by muscular strnctore, and the 

 .pacity of which i capable of great alteration by moseolar 

 agency. Likewise we mu*t remember that in the healthy living 

 xxly no such thing as the cavity of the thorax exists ; the longs 

 and heart completely fill op this apace, and are in close relation 

 a its walla in every part. The remit of these arrangement* is 

 .hat when by any means the capacity of the cheat is ^'miiTfa K +it t 

 air ia driven ont of the longa, and when the pressure is removed 

 ;he lungs by their elasticity expand and follow the walls of the 

 thorax, and so create a vacuum in some of the air-cells, and the 

 atmospheric air at once rushes in through the windpipe to fill 

 ;ho empty cells. During inspiration, the capacity of the chest 

 is, as a rule, increased in every direction, bot the way in which 

 ihis increase is obtained varies in different instances. In young 

 children the act of inspiration ia performed almost entirely by 

 the diaphragm, which, descending, forces down the contents of 

 the abdomen, and so increases the size of the chest. In the 

 adult, in addition to the diaphragm, which still performs a large 

 part of the work, the elevation of the riba by the numerous 

 muscles attached to them comes into play ; and in consequence 

 of the way in which the ribs are articulated with the spine, and 

 their cartilages with the breastbone, making the centre of the 

 rib the lowest point, any raising of the riba at the same time 

 draws them outwards, and the ends being both more or less 

 fixed tends to bring the ribs into nearly a straight line with 

 the cartilages, and so, as a matter of course, enlarges in a very 

 marked degree the capacity of the chest. This action will be 

 at once understood, if reference be mode to the illustration of 

 the thorax, given in one of the earlier lessons on this subject. 

 The chest and lungs during expiration resume their ordinary 

 size by reason of their elasticity, which in deep expiration is 

 aided by the abdominal muscles contracting and forcing up the 

 diaphragm, which remains passive during expiration. 



The quantity of air changed at each inspiration varies in 

 different people, and this variation has been taken as a measure 

 or index of the physical strength and constitution of the indi- 

 vidual. Thus it has been found by experiment that a healthy 

 man five feet seven inches in height can expire 225 cubic inches- 

 of air, and that for every additional inch of stature an increase 

 of eight cubic inches in the capacity takes place. This rule ia 

 not much affected by the weight of the person, but age is found 

 to modify it to a certain extent ; thus the capacity increases 

 from about the fifteenth to the thirty-fifth year, and then 

 gradually diminishes. The number of respirations in the- 

 minute is, on an average, from fourteen to eighteen in a state of 

 repose of body and mind ; but this is liable to great variation 

 from disease, mental emotion, or physical exertion. 



The purpose of this function of respiration is to submit the 

 blood charged with the waste material of the body to the 

 purifying action of the air ; from this contact of the blood with 

 the air, certain changes ore induced in both the blood and the 

 air ; these must now be examined, and, as a preliminary, we 

 must stop for a minute, and see of what the atmospheric air is 

 composed. In almost all positions the composition of the air 

 is identical, and for our present purpose it will be enough to 

 say that it contains oxygen, nitrogen, carbonic acid, and 

 watery vapour. There is in it about 21 per cent, of oxygen 

 to 79 per cent, of nitrogen by measure, or 23 per cent, of 

 oxygen to 77 per cent, of nitrogen by weight. The quantity of 

 carbonic acid is very small, not more than 4 to 5 parts in 

 10,000. The quantity of water in a state of vapour varies 

 greatly, being influenced by temperature and other causes ; bot 

 it is never entirely absent from the atmosphere. The changes 

 which take place in the air during respiration are as follow : 

 First, the oxygen is diminished ; secondly, the carbonic acid is 

 increased ; thirdly, the temperature is raised ; fourthly, the 

 moisture is increased. Of these changes the first two are by 

 far the most important, and may be considered together, as one 

 is in a great measure dependent on the other. The oxygen ia 

 diminished, because it is absorbed, and enters into combina- 

 tion with the surplus carbon of the system, to form carbonic 

 acid not that the whole of the oxygen absorbed is utilised in 

 this manner ; some of it, doubtless, assists in forming some of the 

 other compounds carried out of the body by means of the skin 

 and kidneys. The quantity of oxygen absorbed varies with 

 different circumstances and in different individuals. Animals 

 of a small size consume a much larger quantity in proportion to 

 their size than larger ones. The kind of food on which an 



