With this view the exact description of Mr. Rainey coincides :—" In the mammal the number of capillary plexuses is not, as some have supposed, the same as that of the air cells ; that is to say, a terminal artery does not divide into a plexus at any particular part of a cell, its branches uniting for the com mencement of a vein on the opposite part. On the contrary, one plexus passes between and supplies several cells. In the interior of the lung the exact extent of an individual plexus cannot be determined, in consequence of the removal of some part of it by the sec tion necessary for its exhibition. But, on the surface of the lung, where the extent of these plexuses, in relation to the cells over which they ramify, can be easily rnade out, an individual plexus may be seen to spread over an area of ten or twelve cells in some parts and in fewer on others, the exact num ber depending in some measure upon the size of the cells." * Around the foramina and margins of the cells very frequent anastomosis takes place be tween the minute branches of the pulmonary artery. With reference to this artery, it should, however, be stated, that it differs from all other arteries in the extremely infrequent inoscula lions which occur between its secondary and tertiary branches—and that its blood mingles with that of no other vessel ; it is poured en tire into the pulmonary veins. " The trunk" of a vessel is most certainly very- seldom seen on the flat expansion of a cell-wall. But it is quite an error to suppose that each cellule has its own separate arterial ramuscule. Professors Harting and S. Van der Kolk's injections place this point beyond doubt : — " quod ad divisionem ramorum arteriarum at tinet, animadvertendum est, non alveolum quemque singulum ramulum accipere, quum hoc tantum valeat de infundibulis, ita ut lobulus, infundibula continens, accipiat trunctun arte ille truncus ad numerum infundibulorum dividatur et iterum subdividatur." (Adriani). From this description it results that the ca pillary web without an intervening trunk stretches from cell to cell. One more lin portant fact remains to be stated with refer ence to the capillary plexus. it is nowhere doubkd upon itself, as it is in the lung of the reptile. Every cell-wall and every partition between the cells bears only a singk layer of vessels. The opposed sides of such a layer must therefore bound two different cells, and the current of blood by which it is traversed must be subjected on both its flat sides to the action of air contained in the cells. If this plexus were double, only one side could re ceive the influence of the atmosphere. This type is exemplified in the reptilian lung. All other thing,s being equal the respiration of the mammal and man must be twice in amount that of the reptile. But this anato mical fact has also an interesting pathological bearing. When the capillary layer, or rather the blood borne by it, is the seat of disease, the products of that disease must be poured into the two contiguous cells, between which it is interposed, at the sante time and in the same amount. This accounts for the rapidity with which pneumonic infiltration occurs. In the,most injected preparation the diameter of the capillary vessels of the ?vie mirabile exceeds that of the meshes. This, however, is not an exact expression of their propor tions in the living state. The diameter of a single capillary, measured in the injected state on the wall of an air-cell, does not exceed Thy (Todd and Bowman.) The human red blood corpuscle ranges, in the same drop of blood, from ze,ny to of an inch in dia meter, the average being from tO Then two red-corpuscles may traverse any 'single capillary abreast ? When allowance is made for the difference between the internal and external diameters of the vessel, it will appear very probable that in man and mam mals only a single row of red-corpuscles tra verse the pulmonary capillaries at a time. In the lung of the reptile it is to be proved by observation of the living circulation, that a double row of corpuscles really does move along the vessels. This fact must reduce the amount of oxygenation which in a given time a single corpuscle receives.
The puhnonary Veins convey the blood, arterialised in the plexus just described, to the left auricle. The distribution of the pul monary veins differs strikingly from that of the artery and bronchia. Each lobule has its separate arterial and bronchial branch. This
regularity does not obtain with respect to the veins. The puhnonary veins arise in the form of minute radicles in the capillary plexus of the air-cells. Now although, as formerly stated, the individual air-cells are not furnished with a separate arteriole and venule, the extreme branches of the pulmonary artery- anti the in cipient venules are separated in different parts of the lung by areas of similar dimensions. It follows, therefore, that the tinze during which a globule of blood in different parts of the lung is exposed to the agency of the air is equal,; in other words, every drop of blood which enters the lung is arterialised to the sanie amount from the equality of the areas of exposure over which it passes. It should also be observed that as the capillary web is spread over several cells, every' particle of blood in lransilit from artery to vein traverses the circumferences of several cells. This is a beautiful provision for securing certainty to a vital process. The minute venules unite to form visible trunks, which course irregularly over and between the cells and intercellular passages. They observe a general diagonal direction, the bronchia and artery occupying with great constancy the geometrical axes of the lobule. They emerge out of this lobular space not at its apex, in company with the air-tube and artery, but at every or any point of its circumference. This is more obviously the case as regards the lobular veins along the surface of the lung. Those more centrally si tuated follow the bronchia more closely. This explains why Reisseissen, Cruveilhier, and others have expressed very opposite opinions upon this point. In the intralobular spaces the veins proceeding from several lobules unite together into a trunk common to them all. The larger trunks, resulting from the conflu ence of the smaller, converge towards the roots of the lungs, but by a route different from that of the bronchia and arteries. Thus the general mass of the lung may be-regarded as containing two series of ramified canals; one transmitting the bronchial tubes, the nerves and pulmonary artery, the other the pulmo nary veins. This interesting fact was well described by Dr. Addison of Guy's Hospital in a paper in the Medico Chirurgical Trans actions in 1840. At the root of the lungs four pulmonary veins result, which discharge their blood into the left auricle. " The cause of the separate course of the pulmonary arteries and veins is to be found in the opposite posi tion of their radicles in reg,ard to the capillary net-work of the lobules, it being a convenient arrangement for the terminal arterial and venous twigs to hold alternate positions among the capillary net-work, so that each arterial twig dispenses its blood in all direc tions, and each venous radicle collects it from all sides." * The Bronchial System of Vessels consists of arteries and veins. The bronchial arteries are commonly described as the nutrient vessels of the bronchial tubes. They arise from the front of the descending or thoracic aorta. They are, however, variable in number as well as in place of origin. They are commonly described as the inferior and superior.
The superior, two usually in number, arise either separately or by a common trunk from the front of the aorta, opposite the third or the fourth dorsal vertebra, and one directed to each side adheres to the posterior surface of the bronchial tube, on which it divides into branches, and passes into the interior of the lung.
The inferior, two or more in number, arise lower down than the preceding, and are distri buted, like them, on the bronchus of each lung : these small arteries give branches to the cesophagus, bronchial glands and pericar dium. The superior bronchial artery of the left side may arise from the superior inter costal artery. Every successful injection ex hibits large and long branches from these ves sels, leaving the tracks of the bronchi, and entering into the inter-lobular tissue and sub-pleural tissue. -I The bronchial Veins accompany the arteries, and the branches unite one for each side ; the right opens into the azygos vein, and the left into the superior intercostal vein. Many branches may be traced also to the cesopha geal veins, and those of the posterior medias tinum. Numerous branches of these veins may be observed to wander under the pleura, in the sub-pleural tissue.