476 EESPIEATION right ventricle and run through the lung substance pari passu with the bronchial tubes to the lobules. Here they branch into a dense network of capillaries which spread over the outside of the alveoli, enmeshing them so tightly as to indent their walls. From the capillaries the blood Hows into pulmonary venules which run together to form the large pulmonary veins, which open into the left auricle. The pulmonary venules may anastomose freely, but the arteries never do so. The veins possess no valves. The lymphatic vessels of the lung abound in all parts, but are usually described as having a threefold distribution, (a) in the layer of tissue beneath the membrane investing the whole lung, (6) in" the perivascular tissue, and (c) in the peribronchial tissue. When fine carbon particles are introduced into the alveoli of the lung they find their way with the greatest ease into the inter- alveolar tissues, and finally come to lie in the three positions just referred to, as may be demonstrated in the lung of any coal miner. The lymphatics of the membrane investing the lung communicate with the free surface of that membrane by means of openings not unlike the stomata of leaves. The lungs, with the heart and great vessels, are the chief organs contained in the thorax, or that division of the great body cavity which lies above the diaphragm. Each lung is invested with a membrane called pleura, which plays a most important part in the mechanism of respiration. The Pleural Membrane. Let us imagine a bag shaped like an hour glass, and let us suppose one half of this bag to be packed up small and pushed through the constricted portion of the hour glass into the interior of the other half. We should then have a bag shaped like one half of an hour glass but provided with double walls a more or less globular double-walled bag with a narrow opening into it. If further we imagine the interior of this double-walled bag (not the interior of the hour glass) to be entirely filled with one lung, we shall have an exact conception of the relationship of the lung to the pleura. The lung so covered is placed in its proper half of the thorax, which in circumstances of health it accurately fills. The outer layer of the double-walled bag is closely glued to the inner wall of the thorax. The inner layer is as firmly adherent to the surface of the lung. The space between the double walls, i.e., the interior of the original hour glass, is called the pleural cavity ; it is vacuous in health, being moistened by a mere trace of fluid. The substance of the pleural membrane is mainly connective tissue ; and the interior of the pleural cavity is lined with a single layer of flat epithelial cells exhibiting the stomata already referred to. The Thorax. The chest or thorax is formed by the dorsal section of the spinal column behind, with the ribs that spring from it on each side, and the sternum, which lies between the ends of the ribs, in front. The dorsal spine is bowed, so that its concavity looks forward. The ribs, speaking generally, are bowed with their con- cavity turned towards the interior of the chest ; and if we consider the plane of each rib, i.e., that plane in which the arched rib would (approximately) lie flat, we shall find that it declines from the hori- zontal in a twofold manner first the rib-plane slopes from behind downwards and forwards, and secondly it slopes on each side from the mesial plane of the body downwards and outwards. The ribs 1-7 are connected with the sternum by means of pieces of cartilage which really form the anterior portion of each rib arch ; these ribs are called " true " ; the eighth, ninth, and tenth ribs are united by cartilage, not to the sternum, but to the cartilages of the seventh, eighth, and ninth ribs respectively ; these ribs are called "false" ; the eleventh and twelfth ribs are called "floating " because they are unattached anteriorly. Each rib has a head, by which it is joined to the vertebral bodies constituting the spinal column ; a tubercle or shoulder at a little distance away from the head, by which in all cases, except those of the tenth and eleventh ribs, it is joined to the transverse process of a vertebra ; an angle or rough line a little beyond the tubercle, where the rib, rather suddenly, begins to sweep forwards ; and a neck, the part intervening between the head and tubercle. The space between the ribs is filled up by two layers of muscles called intercostal an outer or superficial layer, whose fibres run from above downwards and forwards, and a deeper or internal layer, whose fibres cross those of the former. The outer layer is not fouud between the costal cartilages in front, nor the inner layer between the costal necks behind. The upper opening of the thorax is filled by the windpipe blood-vessels and other structures pass- ing into or out of the thorax. The floor of the thorax is formed ty The Diaphragm. This consists of a thin arched muscular partition, whose fibres spring from the edge of the lower opening of the thorax and converge towards a sheet of tendon in the. centre, which is shaped somewhat like a trefoil leaf. We may group the muscular fibres of the diaphragm according to the quadrant from which tlu-y spring : (1) a vertebral portion, whose fibres stretch down to be attached in two well-marked columns or pillars to the bodies of some of the lumber vertebrae and by tendinous arches to the transverse processes of the first lumbar vertebra and the twelfth rib ; (2) a sternal portion, which springs from the back of the tip of the sternum, and from the sheath of the rectus abdominis muscles below it ; (3) and (4) two lateral or costal portions, which spring from the lower edge of the thorax all round from the tip of the twelfth rib to the junction between the sixth and seventh costal cartilage where the sternal portion begins. The whole diaphragm forms a dome or cu- pola projecting so far into the thorax that the lateral vertical portions of the dome lie in close apposition to the walls of the thorax, as is shown in fig. 2. The top of the dome is somewhat flat and the right moiety of the top is on a higher level than the left, the highest point corresponding with the level of the junction of the right fifth rib with the sternum. Other Muscles of Re- spiration. The ribs are movable in the sense that each rib plane, which has been described as declining in two ways from the horizontal plane, may be made to approach the horizontal, and may afterwards _ re- F, O . 2. Section through tip of 12th rib. turn to its original (From Hermann.) position. To accomplish these movements various muscles are provided, the exact position of which need not be very fully de- scribed. Suffice it to say that in general they arise from the verte- bral axis, or from some extra-thoracic fixed point, and take hold of the movable parts of the thorax in such a manner that they can pull them up or pull them down. Such muscles are the following (enumerated without reference to their function as inspiratory or expiratory muscles ; particular descriptions of them must be sought in the article ANATOMY) : levatores costarum ; the three scalene muscles ; the sterno-cleidomastoid ; serratus posticns superior ; parts of the great erector spinfe muscle ; possibly serratus magmis ; pectoralis minor and major in certain positions of the arm ; obli- quus externus ; obliquus interims ; transversalis abdominis ; trian- gularis sterni ; serratus posticus inferior ; quadratus lumboruin. In addition to these muscles many others may lend occasional aid in respiration by fixing points, such as the scapula, otherwise too freely movable to act as a point d'appui ; such are trapezius, latis- simus dorsi, rhomboidei, and levator anguli scapuli. Dimensions of the Thorax. The circumference of the chest just below the level of the arms is about 34J inches in men, and 32 inches in women. At the level of the tip of the sternum it is 32 and 30^ inches respectively. The measurement from clavicle to lower edge of thorax varies very much in different cases. The transverse diameter above the nipple is about 10 to 10^ inches in men, and about 9 to 9| inches in women. The antero-posterior diameter, measured from the spines of the vertebra behind to the surface of the chest in front is in the upper part of the chest about 6 inches, and in the lower 7^ inches. The right half of the chest is generally somewhat larger than the left, because its muscles are usually better developed. Instruments .for measuring the exact circumference of the chest at a given level are called cyrtometers ; the best of these is a strip of lead which can be laid along the side of the chest from the spine round to the sternum, and which is pliable enough to follow the inequalities of the chest wall, yet resisteut enough to keep the shape imparted to it. For other figures illustrating the structure of thorax and lungs, see ANATOMY. The Physiological Actions of the Respiratory Organs. The organs above described perform during the whole of life certain movements. The commonest are the move- ments of ordinary quiet respiration, but we must include under the head of physiological actions, as distinguished from those provoked by disease, other movements, viz., forced respiration, such as accompanies strenuous muscular exertion, and those modifications of the respiratory act called coughing, hawking, sneezing, snoring, crying, sigh- ing, laughing, yawning, and hiccoughing.
