84

Structure of the Lungs.

from a subject about fifty years of age, their form seemed not to be as regular or uniform as in the preceding instance: their size varied from 3 to 7 of an inch. The extent of the internal surface of the lung is relatively less in proportion as the air-cells are larger and less numerous.

The pulmonary artery divides into a branch to each lung, which subdivides into branches for each lobe, and for each lobule of a lung. These vessels are accompanied by similar ramifications of the pulmonary veins. In the root of each lung the artery with the veins before and below it extends transversely outwards. The bronchus descends obliquely behind the blood vessels. If coloured water be thrown into the pulmonary artery, it passes into the pulmonary veins, and in part escapes into the bronchi. In the lung of the turtle the air-cells are remarkably large and irregular in their figure; and after a successful injection their surface is found to be reddened with capillary vessels containing size and vermilion. In the lung of a frog the course of the blood in single files from the arteries into the veins may be seen with a microscope. The flow of the blood through the lungs may be considered easier than in any other part of the body, in as far as the atmospheric pressure upon the vessels is counteracted to a trifling degree by the resilience of the texture of the lung.

Each lung receives two or three vessels from the aorta termed bronchial arteries, which are distributed with the bronchi. The pulmonary nerves are derived from the nervi vagi, which pass behind the root of each lung, and throw a plexus of branches round it: their final distribution has not been traced. The lymphatic vessels from the substance and superficies of the lungs are

Cause of the Entrance of Air into the Lungs. 85

received into a vast number of conglobate glands disposed around the bronchi and the bifurcation of the trachea. They are remarkable for their black colour, which increases with age. The lung itself, of a pink colour in infancy, gradually becomes mottled with black. The dark hue in each case is supposed to result from carbon disengaged in the substance of the lungs.

Through the windpipe atmospheric air finds its way into the cells of the lungs: it is inhaled at the instant after birth, and is continually changed and replaced by fresh draughts through the operation of muscles, which alternately expand and contract the cavity of the chest as long as life remains. If the lungs were inextensible and of a sufficiently firm texture, and the muscles which enlarge the chest were to act with unlimited force, a vacuum would be formed between the pleura pulmonalis and the pleura reflexa at each attempt to inspire, and no air would enter the lungs. But as the lungs are readily extensible, atmospheric air rushes into and dilates their cells in exact proportion to the expansion of the area of the chest, and holds the two surfaces of the serous membrane in strict contact :-yet the same points are not always in apposition: when the chest enlarges, the surface of the lung during its expansion slides upon the pleura reflexa, as is shown by the elongation of the shreds of lymph by which the two layers of pleura are often found joined together after inflammation.

The passage of air into the lungs is so free, that the muscles which dilate the chest are not opposed by the atmospheric pressure in a greater degree than those which move the limbs; but they have to overcome the resilience of the lungs, the elasticity of the abdominal

86

Different Degrees of Inspiration.

parietes, and the resistance of the joints of the ribs, which all favour the state of expiration.

The term breathing or respiration includes both the mechanical operation of renewing the air within the lungs, and the changes to which its presence there contributes.

The mechanism, by which the chest admits of being alternately enlarged and diminished, has already been described: every provision which it contains is employed in a greater or less degree at every repetition of breathing. The difference between a moderate and a deep inspiration is in the extent only, to which the diaphragm and the muscles that elevate the ribs contract. But it may be observed, that for the fullest enlargement of the chest, the scapula and clavicle are raised and carried backward by the trapezius, levator scapula, and rhomboïd muscles, so as to give greater effect to the action of the serratus magnus and pectoralis minor; and that to yield a freer passage to the air, the nostrils are dilated, the larynx descends, and the rima glottidis is enlarged. During each expiration the rima glottidis is narrowed. Ordinary breathing takes place between the limits of forced inspiration on the one hand, and forced expiration on the other.

Numerous experiments have been made to ascertain the quantity of air alternately drawn into and thrown out of the chest, in ordinary breathing. Those of Dr. Menzies, which coincide nearly in their result with the researches of Jurin and Fontana, are commonly esteemed deserving of credit; but they differ remarkably from the observations of Sir H. Davy and of Messrs. Allen and Pepys. Differences in the relative size of the thorax in different persons, a difference in the frequency with

Volume of Air inhaled at a single Inspiration. 87

which breathing is performed, and perhaps other causes may have combined to produce this discrepancy. The frequency of respiration ranges between fourteen and twenty-seven times in a minute, but appears commonly to be from seventeen to twenty.

Dr. Menzies employed two processes in estimating the quantity of air habitually inspired. A healthy man five feet eight inches in height, and somewhat more than three feet about the chest, stood immersed in warm water to above his breast, in a vessel which narrowing at the upper part allowed of an accurate estimate of the level to which the water alternately rose and fell while he breathed. His pulse both before and after immersion beat sixty-four or sixty-five, and his respirations were fourteen or fourteen and a half in the space of a minute; and they continued the same during the two hours and upwards that he remained in the vessel without suffering inconvenience. The quantity of air thrown out at each expiration averaged at 46.76 cubic inches. The same person afterwards was employed to fill a cow's allantoïd, a membranous sac well calculated for such a purpose, by repeated expirations. The allantoïd was found to contain 2700 cubic inches of air, and was filled in many trials with fifty-eight expirations, which gives 46.55 cubic inches as the quantity of air expired each time. The same trials repeated upon a man five feet and an inch in height, whose pulse beat seventy-two, and the number of whose respirations was eighteen in a minute, gave from thirty-eight to forty cubic inches as the measure of a common expiration. Repeating the experiment himself, Dr. Menzies filled an allantoïd containing

88

Average Contents of the Lungs.

2400 cubic inches by about fifty-six expirations, giving 42.8 cubic inches as the average quantity of each; and found that he exhausted the allantoïd, when previously filled with atmospheric air, by an equal number of inspirations. Sir H. Davy estimates the quantity of a single inspiration at thirteen or seventeen cubic inches; Messrs. Allen and Pepys at sixteen and a half; Mr. Kite at seventeen; Mr. Abernethy at twelve.

Dr. Menzies observed that many individuals were capable by a forced expiration of throwing out an additional seventy cubic inches; and that the difference between an extreme inspiration and an extreme expiration often exceeded two hundred cubic inches. The lungs after death under ordinary circumstances are probably reduced to the same compass as by a forced expiration during life. Messrs. Allen and Pepys found that the lungs of a stout man about five feet eight inches high after death contained nearly one hundred cubic inches of air. Of this quantity 31.58 cubic inches were expelled by the resilience of the lungs upon opening the thoraxb.

Dr. Bostock estimates the quantity of air, which may be voluntarily expelled from the lungs after an ordinary expiration, at 160 or 170 cubic inches, from trials made upon himself and others. Adding to this quantity 120 cubic inches for the residual air in the lungs, he supposes 290 cubic inches to be the entire contents of the lungs in their natural state, to which about forty cubic inches