Although the cylindrical form is pretty general throughout the arterial system, it is by no means accurately preserved. Several arteries increase in size in the progress of their course ; of this we have examples in the umbilical arteries, which expand they approach the placenta, and the spermatic arteries, especially in the bull and wild boar, which enlarge con siderably as they proceed to their destination. Moreover, Haller§ and Martinus have shown by experiments, that in every instance where an artery divides in the human body, it undergoes a dilatation immediately before such division ; and this fact derives confirmation from the experiments of Mr. Hunter on the carotid arte ries : it is much more unusual for an artery to diminish in size in its course unless it has furnished branches. Santorini states, how ever, that he observed the carotid artery of an ostrich (Struthio camelus) to have become nar ro .ver in a portion of its course of six inches in length, for which space no branch had been given off.
The arteries become smaller and more nu merous by repeated divisions : the combined area of the branches of each artery, however, exceeds the area of the trunk from which they are given off, in every instance, in consequence of which the capacity of the arterial system, as a whole, is increased in proportion to the number of its divisions. It is from this cir cumstance that the arteries have been said to represent a cone, the apex of which is at the heart, and the base in the capillaries.
When an artery divides into several branches of unequal size, the largest usually continues its course in the direction of the original trunk.
The branches of the arteries are for the most part given off at acute angles ; some few, as the superior aortic intercostals, go off at obtuse angles, and the lumbar arteries arise from the aorta at right angles.
The arteries appear in general to take the shortest course to the parts they supply ; hence the tendency they have to run in straight lines.
In many situations the arteries are remark able for having a tortuous course, as is par ticularly evident in the arteries of the stomach, intestines, bladder, uterus, lips, iris, &c., where this disposition appears to be a provision to obviate any interruption to the circulation which might result from the great or sudden changes of volume, form, or situation to which those organs are subject in the performance of their functions : in other instances the arteries appear to be contorted for the purpose of breaking the impulse of the systole of the ven tricle on the blood, and thereby moderating the force with which that fluid is propelled into vessels partaking of the delicacy of structure of certain organs to which they are distributed, as the arteries of the brain, spleen, testicle, &c.
The smaller arteries, running among loose structures, are rendered tortuous during each systole of the ventricle of the heart, a pheno menon which we have frequently witnessed where such vessels were exposed for a few inches Of their course during surgical operations.
Anastomoses.—The several parts of the arte rial system communicate freely with each other; and these communications, known by the name of anastomoses, 1- are more frequent between the arteries in proportion to the remoteness of these vessels from the heart. Three kinds of anas tomosis have been distinguished by anatomists : first, two vessels of nearly equal size approach and join so as to form an arch in such a man ner as to render it impossible to determine the exact point of their union : this arch gives off smaller vessels. Of this kind is the anasto mosis which takes place between the arteries of the intestines and the arteries in the neigh bourhood of joints. Secondly, two arteries are sometimes connected by a transverse branch, as the two anterior cerebral in the arterial circle at the base of the brain. We find this kind of communication, also, between the two um bilical arteries as they approach the placenta. Thirdly, two arteries join at an acute angle, so as to form a single trunk : thus the two verte bral arteries form the basilar, the two anterior arteries of the spinal cord unite in a single trunk ; and in the feetus the ductus arteriosus joins the thoracic aorta in a similar manner. Besides these more obvious communications between vessels of a larger size, the anastomoses of the capillaries are so frequent as to give to those vessels, when successfully injected, the appearance of a fine net-work.
It is by means of the anastomoses that the circulation is carried on in a limb after the trunk of its chief artery has been obliterated by disease, injury, or a surgical operation ; and the well-known efficiency of the anastomosis of arteries in re-establishing the circulation in parts from which the direct supply of blood through the principal artery has been cut off, has led to the performance of some of the most brilliant operations by which modern surgery has been raised to the exalted rank it holds at the present day.