Heart alone not sufficient to circulate the blood. Variations of the supply of ferent organs. functions become disturbed, and various alterations neces sarily take place in the distribution of the blood. If, for example, from any alteration of structure, the blood cannot readily pass from the right auricle into the right ventricle, or from the right ventricle into the pulmonary artery, it will be naturally anticipated that either of these changes will produce some irregularity in the distribution of the venous blood. So, also, when, if either the left auricle or the left ventricle cannot propel the blood in its natural current, or with its usual velocity, a variety of symptoms will arise from a disturbance in the circulation of the arterial blood. On contemplating the structure and function of the central organ of the circulation, for the purpose of elucidating the nature of its various diseases, it is essential that our researches should not be limited to the function of the heart alone, but that such inquiries should be extended to those organs which are intimately associated with the heart, and which materially contribute to the circulation of the blood, Though it may be strictly true that the blood flows in a eircle, and that the heart, like a syringe, has sufficient muscular power to propel the vital fluid throughout the whole system, there are other physical conditions necessary for the due performance of that important function-conditions to which we must constantly refer in all our pathological researches. It is not only indispensable that a certain quantity of blood to dif- blood be sent to every part of the body, but that the supply to the different organs be variously modified. In some it is requisite that such supply be more equal and uniform, whilst in others it is necessary that on certain occasions the quantity of the vital fluid be either diminished or increased. Supply to the brain uniform; to the sto mach varies; t Of the first of these conditions there is an example in the brain, for the due performance of whose functions the supply of blood must be uniform; and of the other we have an illustration in the stomach, to which viscus, during the process of digestion, the quantity of blood is more or less increased. Another illustration of a temporary change in the quantity erectile of blood supplied to particular organs is afforded in the erectile tissues, those structures being employed in certain parts and to the for the sole purpose of altering their form, in order to enable tissues. the organs to which they belong to perform a particular function. Hence the erectile tissue of the nipple becomes injected with blood, to enable the infant to withdraw the milk from the lacteal tubes. Organs entployed in the circulation heart. For thus modifying the distribution of the blood in the various organs, we shall find that, besides the influence of the nervous system, there are employed no less than five other organs as well as the heart: the arteries, the capillaries, the veins, the muscles or active organs of motion, and the respiratory apparatus. The co-operation of these different organs for carrying on the circulation of the blood when the body is in health renders it essential, in prosecuting our pathological researches, that the share which each of them has in executing that important function should be separately considered; for it is evident that, as the healthy circulation depends on the due performance of the natural functions of these different organs, so it will happen that disorders of the heart must modify their functions; and, on the other hand, changes in their functions will influence the functions of the heart. besides the the arteries. The arteries, whether by an elastic power alone, or by Function of elasticity and muscularity combined, lend the heart an important aid in conveying the blood to the capillaries. I have already stated that it is not requisite that the blood should be transmitted to all parts of the body with the same velocity, or that the supply to the different organs be at all times uniform. These purposes, as well as to provide for the exigencies which certain organs may demand, when, from our habits or avocations, the circulation becomes disturbed, are all fulfilled by particular modes in the distribution of the arteries. And the coats of the arteries are beautifully contrived by their elasticity to admit either of an increase or of a diminution in their calibre, in order to accommodate themselves to the constant changes which are taking place in the currents of the vital stream. Differin their modes of dis tribution. Differences in their length. Their tortuosities. Their anastoinoses. As, therefore, the heart, though it has the power to circulate the blood, has not the power to direct any extra quantity of blood more to one part of the body than to another, that office must be performed by changes in the functions of the proper vessels of the organs,-their vasa propria, -influenced and regulated by the nervous system. The length of different arterial trunks,—the different angles at which the branches leave the trunks,-the varieties in the course of arteries, the tortuosities, the different modes in which they ramify,—the anastomoses and plexuses of arteries, are all peculiarities of arrangement which we find in the arteries, and which are calculated to modify the circulation of the blood in particular organs. A remarkable difference in the length of arteries, as well as in the angle by which they come off from the trunk, is exemplified in the renal and spermatic vessels; whilst the carotids and vertebrals, the arteries of the uterus and of the spleen, are each examples of tortuosities in the distribution of arteries, obviously intended either to modify the momentum of the blood, or to ensure a regular supply to particular organs. The anastomoses of arteries, such as that formed by the coronary vessels of the heart, stomach, and lips, and by the arteries of the iris, and likewise the free inosculation between the carotids and vertebrals established by the basilar, are all modifications in the distribution of arteries, which serve the essential purpose of preventing any interruption to the See Appendix necessary supply of blood to the respective organs. (D.) teries. And, although the heart propels the blood with a certain velocity to the smaller branches of the arteries, the modifications in the circulation produced by their numerous anastomoses in many parts of the body must diminish to a greater or less degree the blood's momentum. Plexus of ar The arterial plexus found in man, and which is still more remarkable in some organs of the lower animals, is a mode of arteries ramifying which fulfils an important office in the circulation. The Rete The plexus of the ophthalmic artery in some birds preHovius. vents the pressure of the column of blood on the visual Mirabile of organs when these animals are under water with their heads placed downwards in search of food. Fig. 3. (a, a) Branches of the carotid artery, from which is derived the plexus. (b) The plexus, of a triangular form. From its base innumerable small branches arise, and which unite at the opposite angle (c).-HoOVIUS. Mirabile of Galen. In the graminivorous animals, The Rete whose heads are kept so constantly in a depending position whilst feeding, a like purpose is served by a plexus of the internal carotid artery. Fig. 4 represents, in a foetus Fig. 4. calf, the plexus formed by principal trunk of the internal carotid artery within the head, and which afterwards divides into branches, resembling those in man.-MONRO. Some aquatic animals, whose habits render them subject to great vicissitudes of pressure, from the depth of the waters in which they occasionally dwell, are also provided with a special apparatus, or rete, in the organs of circulation, which appears to me to be intended to guard them against the pernicious effects of that congestion which would other The intercostal plexus in aquatic animals. See Phil. Tr., 1734. See Histoire Anatomique et Physiolo. gique d'un Organe Vasculaire dans Je Cétace. Some organs supplied with several trunks. system. wise be caused by the stoppage of the circulation in the exterior of the body. In the whale, dolphin, and other cetaceous fishes, the intercostal arteries form a very extensive plexus, these vessels dividing into many tortuous branches, which again reunite; so that when the animal descends into very deep water, as its necessities require it to do, the pressure of the superincumbent medium must diminish, and even retard altogether, the blood's flow into the exterior vessels of the body, causing it to accumulate within the great cavities, more especially the chest. It will then collect in the vascular network, as in a reservoir, from which it will again flow to the surface of the body whenever the animal emerges from the deep. This arterial plexus was first described by John Hunter, and subsequently a more detailed account has been given by Brechet, though he has attributed to it a very different use in the economy from what has now been given. To accomplish a similar purpose in the circulation, some important organs are supplied with vessels from several trunks, as the brain, with its two vertebral and two carotid arteries, the stomach, with its branches from different trunks, and the eye, with branches both from the internal and external carotids. The capillaries, too, co-operate with the heart and arteries in carrying on the circulation, and more especially for regulating and modifying the quantity of blood required for particular organs. At the same time, the capillary circulation is independent of the general circulation, of which we see abundant proofs in the temporary flushes of the cheeks, and in the redness of particular portions of the skin. And, when a finger is inflamed, the diseased action appears to be first limited to the cutaneous capillaries; and only when the inflammation has arrived at a certain degree of intensity does the arterial system, and subsequently the heart itself, become influenced. The functions of the capillaries are greatly modified by the condition of the heart, and the action of the heart is equally modified by the capillaries; and so intimately associated are |