OF TIIE CIRCULATION OF THE BLOOD.
AFTER these remarks upon the nature of contractility After these remarks upon the nature of contractility and sensibility, we must proceed to the consideration of the different functions which depend upon the action of these powers ; and, according to the plan which has been laid down, we shall begin with those operations which arc more particularly referable to the contraction of the mus cular fibre. 01 these, the circulation of the blood, on every account, the first rank. Although the ant mal functions acts as it were in a circle, and arc so inti mately connected together, that the intermission of any one of them is followed by some disturbance of the sys tem, yet the circulation of the blood seems to be that from which all the rest derive their origin, and which is the most essential to the well-being of the .whole. In the more perfect animals, respiration is indeed as necessary for existence as the circulation ; but if we may be allowed the expression, it is, as it were, only incidentally so, be cause by respiration we induce that state of the blood which imparts to the muscles, and to the ventricles of the heart among the rest, their power of contraction. In many of the inferior animals this change in the blood is brought about by a different kind of apparatus, so that the function of respiration is altogether superseded ; but there is no animal, in which we have distinct organs and a vas cular system, where the fluid is not carried about by some thing equivalent to the circulation.
We have already remarked that the circulation is, in respect of time, the first function which we are capable of observing in the young animal during its foetal exist ence. Haller informs us that he was distinctly able to trace the rudiments of the future heart in the chicken during incubation, for some time before he could clearly observe either the brain or the lungs. With respect to the relative importance of the heart and the brain, it may be remarked, that although both of them are equally ne cessary for the functions of the most perfect animals, yet we can easily conceive that simple existence may, for some time, be sustained without the intervention of any of the faculties which originate from the nervous system ; but that the nervous system cannot act for the shortest in terval without a due supply of blood from the the heart, or some analogous organ. Upon the whole, therefore, we
are to regard the heart as the centre of the animal frame, which serves to unite the various functions, however dif ferent in their nature and operations, into one connected vital system.
The organs of circulation may be divided into three parts, the heart, the arteries, and the veins. The heart is a hollow muscle, composed of masses of strong longitudi nal fibres, forming an irregular cone, and leaving an inter nal cavity. The outside of the heart is covered with a strong membrane, and the internal cavity is also lined with the same substance ; the muscular part is copiously supplied with blood-vessels, but it is generally described as possessing few nerves in proportion to its bulk. It is suspended from its base by the great blood-vesSels, which form the trunks of the sanguiferous system, and it is en closed in a membranous purse, called the pericardium.
The interior of the heart is divided into two distinct cavi ties, named ventricles ; and there are two membranous bags at the base of the heart, called auricles, forming in all four separate cells, each of the auricles communicating with its corresponding ventricle, while neither the auri cles nor the ventricles have any direct communication with each other. Although the auricles may be denomi nated membranous bodies, when compared with the ven tricles, yet they are plentifully furnished with muscular fibres, and possess the power of contraction.
The use of the heart is, to receive the blood that is brought to it from the veins, and to propel it along the arteries, and this it accomplishes by the contraction of its fibres, in consequence of which its cavities are diminished in size. But the simple diminution of the cavities, and the pressing out of the blood, would not be sufficient for the purpose of the circulation ; for it is not only necessary that the blood be moved, but that it be moved in the pro per direction. For this purpose the heart is furnished with a curious mechanism of valves, which are attached to the orifices of the ventricles and the arteries, and which are so arranged, that when the heart contracts, and the blood is forced out, the current is necessarily directed in the proper course.