The pericaidnun or general covering of the heart, Iuruls a membranous hag, seemingly detached from the body of the he ai t, except at the roots of the large blood essels, where it is lirml• 4-oilmen-IL:owl from this p.art a thin al, 4.: is rt. !let Led the body of the heart, forming its immediate covering. Tne ticardinni consists 01 two layers, of that next the hirdiuNtinum and diaplit :Iglu is of a fibrous texture, while that nt xt the heart is 55 1111t ll,chat calls a serous mem brane. This latter is SU'Onger than the other, and is smooth and polished on the surface next the heart. Be tween the p, ricardium and the heart there is always found a small quantity of a scrolls or watery fluid, called the water or liquor of the pericardium Opiorpr;c(Irdii). On its peripheral surface the pericardium is closely connected to the m•diastinum and the tendinous part of the diaphragm. Thus, it preserves the heart in a fixed position at its base, while it allows free motion to the body or that organ during its contractions and dilata tions.
The heart or the f(ettis differs in several particulars from that of the adult. The eustachizm valve is more distinct, and proportionally Luger in the the oval hole in the partition between the pulmonie and systemic auricles is open, but is furnished with a valve next the systemic auricle, which prevents the return of the blood into the pulmonic sinus. Between the pul monary artery and the aorta, there is, in the foetus, a large canal, which forms a complete communication be tween the. two arterial trunks, thus admitting most of the blood from the pulmonic ventricle to pass into the aorta. This is called the artcrious canal or duct (ea nails vc1 ductits a•terio8us). Both this canal and the oval hole, are obliterated soon after birth.
The heart is possessed of great irritability, and in the living body its cavities arc continually changing their dimensions, owing to the contraction or the mus cular fibres that compose their sides. Both the auri cles and ventricles contract upon the contained blood at frequent and regular intervals. Both auricles con tract at the same time, and both ventricles at the same time ; but the contraction of the auricles and that of the ventricles takes place alternately ; for while the auricles arc contracting, and thus propelling the blood into the ventricles, these latter are in a dilated state ; while, \then the ventricles are contracting, and thus propelling the blood into the great arterial trunks, the auricles are relaxed. The motion of the ventricles being most con siderable, is that chiefly attended to in speaking of the motion of the heart ; and their contraction is, by medi cal writers, termed the .vystole and their dilatation the damtole of the heart.
Of the .irterees.
We have seen a large trunk rising from each of the ventricles of the heart, and to these trunks we have given the name of art, ry. This name is extended to all
the ramifications of these trunks, till they become so small as to be scarcely visible to the naked eye. The arteries then are those blood-vessels which proceed in a gradual series of smaller and smaller branches from the two great trunks rising from the ventricles, and eo11ve:4 the blond, either to the lungs, or to the system at large. They are cylindrical tubes, with sides (On side I able strength and density, thickest next the trnnks, and it -coming so thin towards the small branches, as to he neatly transparent. They arc divided at some dis tanCe front the larger trunks into innumerable minute ladlle hes, that are dispeesed through tne substance of almost is• organ of the body ; so that when, by a pe culiar process, they are successlully filled with coloured wax, and the animal matter dissolved by corroding liquors, the remaining wax that filled the arterial cavi ties, preserves very nearly the form of the original or gan.
The branches of arteries are given oil' at different directions, according to their situation, or according to the distance or position of the they are des tined to supply. Sometimes, as within the great cavi ties of the body, they come off nearly at right angles with the trunk, while in other situations, as in the limbs, they proceed in a direction, making a more or less acute angle with the trunk. In general, where the trunk of an artery divides into two branches, one of these is larger than the other, and may be considered as the con tinuation of the trunk. The arteries seldom run far in a straight direction, hut are generally either in a waving line, or arc bent at an angle. The serpentine course is the more common. The branches frequently communi cate with each other, forming what are called anadtr, nogrs or inosculation8. These arc' most frequent in the skill and membranes, and are particularly obvious in the mesentery that connects the bowels.
When an artery is cut across its cavity in the living body, the section is always circular, and this form it re tains, though great part. of the contained blood be eva cuated. As the blood flows out, however, the circular opening becomes gradually smaller, from the elastic na ture of the arterial coats, by which the vessel is accom modated to the volume of its contents. Taken gene rally, the whole arterial system is considered by most anatomists as forming a cone, the smallest part of which is next the heart ; so that the ramifications taken toge ther, must be considered as larger than the trunk from 55 Lich they proceed. The cavity of the arteries is per fectly continuous, haying no valves except at the origin of the great trunks.