Anastomoses between the Bronchial and Pul monary Systems of Vessels.— Since the days of Ruysch, Haller, Soemmering, and Reisseissen, this has proved a vexed question in anatomy. One point in this controversy has been over looked. The bronchial arteries are said to be the nutrient vessels, not of the lungs, but of the bronchi. The tissue composing the struc ture of the air-cells and intercellular passages of the lungs is nourished by the blood of the pulmonary system. In the lungs of Reptiles there exists no bronchial system of vessels. The solid walls of these organs are occupied exclusively by the pulmonary system. It is upon the latter, therefore, that the function of nourishing the substance, the parenchynza, of these organs must devolve in these animals. Thus is proved the capacity of this blood. The epithelial particles and elastic fibres; of the air-cells derive the materials of their nu trition from the blood of the plexus (the true pulmonary) on which they immediately lie. It is indisputable, therefore, that the afferent blood of the lungs, like that of every other gland, discharges a twofold office,—that pro per to the gland, and that of nourishing its tissue. Two systems or layers of capillary plexuses are discoverable on the walls of the bronchial tubes ; one lies immediately under neath the mucous membrane, and exhibits ex tended oblong meshes, which run parallel with the yellow elastic fibres; the other lies on the outside of the circular muscular layer, so that the stratum of muscles is interposed be tween the two systems of vessels. This outer layer of vessels, its trunks and capillaries, run circularly with the fibres of the muscles and at right angles with those of the submucous layer. The blood of the former empties itself into the pulmonary vessels ; that of' the latter (the outer) returns by means of the bronchial veins. This, in brief, is the result of the au thor's investigations. They are confirmatory of those of Adriani. The nzode in which the bronchial and pulmonary vessels communicate is stated differently by different authors. Some suppose that the blood of the bronchial arte ries is poured directly into the pulmonary artery, with the venous blood of which it admixes, and like which, traversing the respi ratory plexus, becomes arterialised before it reaches the left auricle. On this supposition the blood enterino. the left auricle would be purely and exclusively arterial. By other ana tomists—of these Rossignol is the most pro minent*—it is contended that the bronchia blood is poured into the pulmonary systern at the left side of the respiratory rete. The cur rent, therefore, entering the left auricle is not pure arterial blood : it is alloyed by the venous rivulet received from the bronchial system, — a reptilian characteristic traceable in human organisation.
By a third class of observers it is said, that the capillaries of the pulmonary and those of the bronchial system of vessels intimately inosculate. The precise solution of this question is difficult, in consequence of the readiness with which an injection thrown into one vessel will pass into another by extrava sation. Other anatomists suppose that the three above-described modes of communica tion actually exist. It is certain that these two systems do communicate, and that only a part of the blood of the bronchial arteries re turns by the bronchial veins. More recently, a new aspect has been given to this controversy by the statements of Dr. Heale, to the effect that the bronchial and the pulmonary systems of vessels do not in any manner or degree communicate. He maintains, on the evidence afforded by his injections, that the vascular web of the air-cells extends, and is prolonged over the internal surfaces of the bronchial tubes. Dr. Heale assigns to this extension of the rete mirabile the power of prolonging the aeration of the blood. This is impossible. The bronchial tubes, the minutest, are inter nally lined by a dense ciliated epithelium. Such epithelium does not exist on the true capillary parts of the lungs of any vertebrated animal. Where there is ciliated epithelium, a universal principle of structure requires in the higher vertebrated animals that the function of breathing should be suppressed. This prin ciple, however, does not obtain in respiratory organs of the invertebrata, and in the bron chial organs of lower vertebrata.
Respiratory Organs of Birds.
The lungs of birds are two in number, symmetrically developed, flattened, and ir regularly triangular in figure. They are fixed, by means of areolar tissue, to the ribs and vertebral column, from the inequalities of which they receive deep impressions. They extend from the second dorsal vertebra as far as the kidneys, and laterally to the junction of the vertebral with the sternal ribs. In their fixed position under the back and near the centre of gravity, they contrast strikingly with the lungs of' mammals, which float loosely in the thoracic chamber. In colour they are blood-red, and in general texture they are more fragile than the lungs of mammals. They are not divided by deep
fissures into lobes, like the mammalian lung ; lobuli, however, exist, although more length ened in form than those of the mammal lung. In the former, as in the latter, a lobule is a smaller lung. All its parts are complete. The lobuli are embraced and isolated by membranes of areolar elastic tissue. A pleural investment embraces their sternal aspects, and an aponeurosis, proceed ing from the diaphragmatic muscles below, blends its fibres with those of this covering. The trachea, after a course in the neck vary ing with the length of this part, at its entrance into the lungs, divides into two primary bronchi, one for each lung. At the place of this bifurcation there exists, in most birds, a complex mechanism of bones and cartilages, moved by appropriate muscles, and consti tuting the true organ of voice. This part is known as the inferior larynx.* The trachea is composed of rings of cartilage which are not deficient at the posterior third of the circle, as in quadrupeds. The successive rings are linked together into a cylindrical form by means of a highly extensile and elastic membrane. The whole cylinder is embraced in a second concentric cylinder of muscular fibre which belongs to the voluntary or striped variety. In this particular it differs from the trachea of mammals. In the latter, only the deficient portion of the rings is composed of muscular fibre, and that too of the involuntary or unstriped kind. This muscular layer in birds extends from the superior larynx to the com mencement of the bronchi : these latter are, however, unsupplied by muscular fibres. They are exclusively membranous.* The bronchi in the case of birds, on entering the substance of the lungs, divide and subdivide without decreasing in diameter (a, a, a, fig. 225.) Patches of cartilage appear in the parietes only of the largest order of these tubes. They are distinguishable into two principal classes: those, first, which course superficially along the inferior or sternal surface, and which terminate by wide openings in the thoracic and abdo minal air-receptacles. This class of tubes is perforated by the inter-cellular passages only on one side, the other being strengthened by cartilaginous semi-rings. The deep bronchi, resembling cylindrical tubes, traverse the lungs in many directions, and freely commu nicate with each other, not, however, to form a network, for they run in nearly pa rallel directions. These tubes are always patulous on dissection, and seem incapanle of contraction and dilatation. They are lined internally by a well-marked ciliated epi thelium. The submucous tissue in the true bronchi is strong and dense, composed chiefly of elastic fibres, none of a muscular character. It constitutes a distinct fibrous layer, like that which lines the trachea of quadrupeds. Those bronchi which do not end in open orifices on the surface of the lung terminate ccecally. These ccecal extremities are perfectly defined by a prominent lining of fibrous and mucous membrane. It was first proved by Mr. Rainey that in the lungs of birds the mucous mem brane does not extend inwards in the direc tion of the interior of the lungs beyond the limits of the bronchi. By the words mucous membrane Mr. Rainey desires to indicate that flocculent covering which is so well seen in his injected preparations. By this observer it is maintained that all parts of the lungs of birds beyond the extremes of the bronchi are literally devoid of all epithelial covering whatever, the extreme capillary vessels being included in nothing but their own proper tunics. It has been already shown that Mr. Rainey has mistaken the cessation of the ciliated epithelium at the ends of the bronchi for the termination of all the other elements of this covering. The apparently naked vessels of the air-cells are really in vested by a hyaline epithelium, coinciding with that which, in the instance of reptiles, will afterwards be described. The abrupt termina tion of the bronchial tubes marks the abrupt commencement of the intercellular passages. These passages contrast remarkably in struc ture with the bronchi. The membranous walls of these parts are reduced to the ut most state of thinness; those of the former are furnished with cylindrical epithelium and a dense fibrous coat. But, what is extra ordinary, the dense mass of vessels which by the latter. Every part of the ference of this current, less than 3-A7 of an inch, is under_the direct agency of the aerat bound these passages are not joined together into a continuous partition. Each vessel is separate from and unconnected with those adjacent. " A wall" thus constructed is at every point between the vessels permeable to air. These "intercellular passages" (b, b, 1, fig. 225.) arise, with singular uniformity, from the sides of the bronchi, at right angles to the axes of the latter. This is so constant as to become a characteristic point of structure in the bird's lung.