GENERAL CONSIDEEATIONS. 601 



their termination, between two collateral branches, no sensible difference ia 

 perceived. 



Mode of Origin. — The arteries are detached in an angnlar manner from 

 the parent branches. Sometimes the an,i(le of separation is more or less acute — 

 this is most fre(iuentlj the case ; sometimes it is at a right angle, and at other 

 times it is obtuse. It will be readily undei-stood that the degree of this angle 

 exercises a somewhat marked influence on the course of the blood. For example, 

 the blood from a principal vessel, in passing into the canal of a secondary one 

 which springs from it at an obtuse angle, must experience a notable check in its 

 im|jetns, because of the change in direction it has to encounter ; on the contrary, 

 the rapidity of the current is not modified to any appreciable degree in those 

 vessels which separate from them at a very acute angle. Towards the point of 

 separation, there is always remarked, in the interior of the vessel, a kind of spur, 

 the sharp border of which is towards the heart — thus dividing the cuiTcnt of 

 blood and diminishing the resistance. This spur resembles in its arrangement 

 the pier of a bridge, against which the watere are divided to pass on each side. 

 (When a short trunk divides abruptly into several branches, proceeding in 

 different directions, it is termed an axis. A very peculiar feature in the division 

 of arteries, however, and one which will be made amply conspicuous in the 

 following description, is their bifurcation or dichotoraous arrangement, which 

 prevails so largely.) 



Course. — In the course pursued by an artery, it is necessary to consider the 

 situation occupied by the vessel, its direction, relations, and the anastomoses which 

 establish communication between it and the neighbouring vessels. 



Situation. — The arteries tend constantly to recede from the superficial parts, 

 and to become lodged in the deeper regions ; in this way they are i-emoved from the 

 hurtful action of external causes — a tendency all the more marked as the arteries 

 are more considerable in volume, but which ceases to be manifested in the less 

 important ramuscules. These vessels, therefore, occupy either the great cavities 

 of the trunk, or the deep interstices on the internal face of the limbs ; when they 

 pass over an articulation, it is always on the side at which flexion occure. But 

 in the limbs, for instance, the joints are flexed alternately in opposite directions, 

 and it then happens that the arteries in these regions have a slightly helicoid (or 

 spiral) arrangement. This is evident in the case of the femoral artery, which 

 passes round the inner face of the femur to become the popliteal artery ; and 

 also in the humeral artery, which is at first situated on the inner side of the 

 scapulo-humeral articulation, then winds around the humerus to be placed in 

 front of the elbow-joint. 



Direction. — The arteries are sometimes rectilinear, and at other times more 

 or less flexuous. The latter disposition is evidently intended to prevent lacera- 

 tion of the vessels in organs capable of elongation and contraction, as may be 

 remarked in the tongue ; or to moderate the impetus of the blood, as in the 

 internal carotid arteries. 



Relations. — In their course, the arteries may be in contact with the viscera, 

 nerves, muscles, bones, skin, and connective tissue. 



a. In nearly every part of the body, the arteries maintain the most intimate 

 relations with the veins — sometimes with two of these vessels, when the artery is 

 placed between them ; sometimes with only one, which is always more supei-ficial. 



b. The arteries are usually accompanied by nerves belonging to the cerebro- 

 Bpinal or sympathetic systems. Those of the latter category are distinguished by 



