ARTERY. 



221 



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- 

 rower 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,! 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- 



* Observationes Anatom. c. 7. n. 6. 

 t From am, per, nopa., os. 



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 fetus 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. 



The larger trunks of arteries are inclosed 

 within the cavities of the body, or run their 

 course on the sides of the limbs least exposed 

 to external injuries, being in general deeply 

 situated in the intervals between the muscles, 

 so as to be protected against wounds or other 

 external injuries, to which they are therefore 

 less exposed than if they had been more super- 

 ficially situated. 



The arteries and their branches are every 

 where surrounded by a layer of cellular tissue, 

 called the arterial sheath, connected more or 

 less intimately with the neighbouring struc- 

 tures, but having so loose an attachment to the 

 arteries as to allow them to glide freely on its 

 inner surface in all their motions, by which 

 means they frequently escape being injured 

 when penetrating wounds traverse parts con- 

 tiguous to them ; and it is owing to the loose- 

 ness of the attachment of the arteries to their 

 sheath that they retract so remarkably within it 

 when cut across. The sheath is generally 

 strongest around the arteries most exposed to 

 external injury : thus it is particularly strong 

 where it surrounds the arteries of the limbs ; it 

 is less distinct on the arteries within the thorax 

 and abdomen, many of which receive coverings 

 from the serous membranes; and it is so ex- 

 tremely delicate around the arteries of the 

 encephalon as to have its existence in this 

 situation questioned by some anatomists. 



Structure of arteries.- The arteries are of a 

 pale buff colour when empty. The absolute 

 thickness of their parietes is greatest in the 

 larger trunks, but more considerable in pro- 

 portion to their calibre in the smaller branches. 



