ON THE FORM AND DISTRIBUTION OF THE VASCULAR SYSTEM AS A WHOLE; ANOMALIES; RAMIFICATION; ANASTOMOSIS.
The double heart. Universal systemic capillary anastomosis. Its division, by the median line, into two great lateral fields—those subdivided into two systems or provinces—viz., pulmonary and systemic. Relation of pulmonary and systemic circulating vessels. Motions of the heart. Circulation of the blood through the lungs and system. Symmetry of the hearts and their vessels. Development of the heart and primary vessels. Their stages of metamorphosis simulating the permanent conditions of the parts in lower animals. The primitive branchial arches undergoing metamorphosis. Completion of these changes. Interpretation of the varieties of form in the heart and primary vessels. Signification of their normal condition. The portal system no exception to the law of vascular symmetry. Signification of the portal system. The liver and spleen as homologous organs,—as parts of the same whole quantity. Cardiac anastomosing vessels. Vasa vasorum. Anastomosing branches of the systemic aorta considered in reference to the operations of arresting by ligature the direct circulation through the arteries of the head, neck, upper limbs, pelvis, and lower limbs. The collateral circulation. Practical observations on the most eligible situations for tying each of the principal vessels, as determined by the greatest number of their anastomosing branches on either side of the ligature, and the largest amount of the collateral circulation that may be thereby carried on for the support of distal parts.
COMMENTARY ON PLATES 1 & 2.
THE FORM OF THE THORACIC CAVITY, AND THE POSITION OF THE LUNGS, HEART, AND LARGER BLOODVESSELS.
In the human body there does not exist any such space as cavity, properly so called. Every space is occupied by its contents. The thoracic space is completely filled by its viscera, which, in mass, take a perfect cast or model of its interior. The thoracic viscera lie so closely to one another, that they respectively influence the form and dimensions of each other. That space which the lungs do not occupy is filled by the heart, &c., and vice versa. The thoracic apparatus causes no vacuum by the acts of either contraction or dilatation. Neither do the lungs or the heart. When any organ, by its process of growth, or by its own functional act, forces a space for itself, it immediately inhabits that space entirely at the expense of neighbouring organs. When the heart dilates, the pulmonary space contracts; and when the thoracic space increases, general space diminishes in the same ratio.
The mechanism of the functions of respiration and circulation consists, during the life of the animal, in a constant oscillatory nisus to produce a vacuum which it never establishes. These vital forces of the respiratory and circulatory organs, so characteristic of the higher classes of animals, are opposed to the general forces of surrounding nature. The former vainly strive to make exception to the irrevocable law, that “nature abhors a vacuum.” This act of opposition between both forces constitutes the respiratory act, and thus the respiratory thoracic being (like a vibrating pendulum) manifests respiratory motion, not as an effort of volition originating solely with itself, but according to the measure of the force of either law; as entity is relationary, so is functionality likewise. The being is functional by relationship; and just as a pendulum is functional, by reason of the counteraction of two opposing forces,—viz., the force of motion and the force of gravity,—so is a thoracic cavity (considering it as a mechanical apparatus) functional by two opposing forces—the vital force and the surrounding physical force. The inspiration of thoracic space is the expiration of general space, and reciprocally.
The thoracic space is a symmetrical enclosure originally, which aftercoming necessities modify and distort in some degree. The spaces occupied by the opposite lungs in the adult body do not exactly correspond as to capacity, O O, Plate 1. Neither is the cardiac space, A E G D, Plate 1, which is traversed by the common median line, symmetrical. The asymmetry of the lungs is mainly owing to the form and position of the heart; for this organ inclines towards the left thoracic side. The left lung is less in capacity than the right, by so much space as the heart occupies in the left pulmonary side. The general form of the thorax is that of a cone, I I N N, Plate 1, bicleft through its perpendicular axis, H M. The line of bicleavage is exactly median, and passes through the centre of the sternum in front, and the centres of the dorsal vertebral behind. Between the dorsal vertebral and the sternum, the line of median cleavage is maintained and sketched out in membrane. This membranous middle is formed by the adjacent sides of the opposite pleural or enveloping bags in which the lungs are enclosed. The heart, A, Plate 1, is developed between these two pleural sacs, F F, and separates them from each other to a distance corresponding to its own size. The adjacent sides of the two pleural sacs are central to the thorax, and form that space which is called mediastinum; the heart is located in this mediastinum, U E, Plate 1. The extent of the thoracic region ranges perpendicularly from the root of the neck, Q, Plate 1, to the roof of the abdomen—viz., the diaphragm, P, transversely from the ribs of one side, I N, Plate 1, to those of the other, and antero-posteriorly from the sternum, H M, to the vertebral column. All this space is pulmonary, except the cardiac or median space, which, in addition to the heart, A, Plate 1, and great bloodvessels, G C B, contains the oesophagus, bronchi, &c. The ribs are the true enclosures of thoracic space, and, generally, in mammalian forms, they fail or degenerate at that region of the trunk which is not pulmonary or respiratory. In human anatomy, a teleological reason is given for this—namely, that of the ribs being mechanically subservient to the function of respiration alone. But the transcendental anatomists interpret this fact otherwise, and refer it to the operation of a higher law of formation.
The capacity of the thorax is influenced by the capacity of the abdomen and its contents. In order to admit of full inspiration and pulmonary expansion, the abdominal viscera recede in the same ratio as the lungs dilate. The diaphragm, P P, Plate 1, or transverse musculo-membranous partition which divides the pulmonary and alimentary cavities, is, by virtue of its situation, as mechanically subservient to the abdomen as to the thorax. And under general notice, it will appear that even the abdominal muscles are as directly related to the respiratory act as those of the thorax. The connexion between functions is as intimate and indissoluble as the connexion between organs in the same body. There can be no more striking proof of the divinity of design than by such revelations as anatomical science everywhere manifests in facts such as this—viz., that each organ serves in most cases a double, and in many a triple purpose, in the animal economy.
The apex of the lung projects into the root of the neck, even to a higher level, Q, Plate 1, than that occupied by the sternal end of the clavicle, K. If the point of a sword were pushed through the neck above the clavicle, at K, Plate 1, it would penetrate the apex of the right lung, where the subclavian artery, Q, Plate 1, arches over it. In connexion with this fact, I may mention it as very probable that the bruit, or continuous murmur which we hear through the stethoscope, in chlorotic females, is caused by the pulsation of the subclavian artery against the top of the lung. The stays or girdle which braces the loins of most women prevents the expansion of the thoracic apparatus, naturally attained by the descent of the diaphragm; and hence, no doubt, the lung will distend inordinately above towards the neck. It is an interesting fact for those anatomists who study the higher generalizations of their science, that at those very localities—viz., the neck and loins, where the lungs by their own natural effort are prone to extend themselves in forced inspiration—happen the “anomalous” creations of cervical and lumbar ribs. The subclavian artery is occasionally complicated by the presence of these costal appendages.