720 HUMAN ANATOMY. 



so that, as a rule, it may be stated that the thickness of the wall of an artery is 

 directly proportional to the calibre of the vessel. Exceptions to the rule exist, how- 

 ever, and the thickness of the wall is not necessarily the same in vessels of identical 

 calibre. 



Another general rule, to which there are also exceptions, is to the effect that the 

 calibre of an artery is proportional to the extent of territory which it supplies. At 

 each point where a branch is given off from an artery a diminution of the calibre 

 occurs, but throughout the interval between successive branches the size of the vessel 

 usually remains unchanged. Where, however, a marked alteration in the direction 

 of an artery occurs, its diameter undergoes a slight diminution, but is re-established, 

 or, indeed, increased for a short distance, so soon as the change of direction is 

 accomplished. These constrictions, which are especially noticeable in large arteries, 

 such as the aortic arch or the subclavian, are termed arterial isthviuses, and the 

 enlargements which succeed are known as arterial spindles. 



The area of the transverse section of a left subclavian artery before any branches were given 

 off was found to be 27.6 sq. mm., that of a section of the isthmus was 15.6 sq. mm., while that 

 of a section taken about 2 cm. beyond the isthmus was 20 sq. mm. In the case of an aorta in 

 which the spindle was well marked, the area of a transverse section of the isthmus was found to 

 be 46 sq. mm., that of a section through the spindle was 65 sq. mm., and that of a section of the 

 thoracic aorta a little below the spindle was again 46 sq. mm. (Stahel). 



THE GENERAL PLAN OF THE ARTERIAL SYSTEM. 



An idea of the general plan of the arterial system may be most readily obtained 

 by reference to the arrangement occurring in the fishes (Fig. 677), in which respira- 

 tion is performed by gills borne upon a series of branchial bars which form the 

 lateral walls of the pharynx. In these forms the heart consists of but two cham- 

 bers, an atrium which receives the great veins and a ventricle from which a single 

 aortic trunk, the trnncus arteriosus, arises. The heart contains only venous blood, 

 and its function is to drive the blood through the gills, where it becomes oxygen- 

 ated, and whence it passes to the various organs of the body. The heart is situated 

 far forward, beneath the posterior portion of the pharynx, and the aorta passes forward 

 from it along the floor of the pharynx, sometimes dividing early into two parallel 

 stems, the ventral aortce. From these, and from the aorta before its di\dsion, 

 branches pass off to each of the gill-arches and, breaking up into capillaries, traverse 

 the gill-filaments borne by the arches. After being oxygenated in the gill-filaments, 

 the blood from each gill is collected again into a stem which joins with those coming 

 from the other gills of the same side of the body to form a longitudinal trunk situated 

 on the roof of the pharynx, and this trunk, passing backward, unites with its fellow 

 of the opposite side to form a dorsal aorta, which is continued throughout the entire 

 length of the body immediately beneath the vertebral column. 



From the forward part of each of the dorsal longitudinal stems branches are 

 continued forward into the head region, and throughout the entire trunk region the 

 dorsal aorta gives of? laterally paired branches corresponding to each of the seg- 

 ments of the trunk, and from its ventral surface one or two series of visceral branches 

 which are also arranged segmentally. 



At one stage in the development of the human embryo the arrangement of the 

 arterial system is essentially the same as that which has just been described, except 

 that, since there are no longer any gill-filaments, the capillaries of the branchial 

 vessels are lacking. By a series of important changes, later to be described (page 

 846), this arrangement is converted into that found in the adult, the relation between 

 the human arrangement and that occurring in the fishes being shown by a comparison 

 of the preceding diagram with Fig. 678. It will be seen that the fourth branchial 

 arch of the left side is represented by the arch of the aorta, the anterior portion 

 of the dorsal aorta becomes what is termed the internal carotid artery, the forward 

 prolongation of the ventral aorta becomes the external carotid artery, and the con- 

 necting link between these two vessels represents the third branchial vessel. And, 

 finally, the last pair of branchial vessels is represented by the pulmonary arteries. 



While the arteries have their primary embryonic arrangement, the heart lies far 

 forward beneath the posterior portion of the pharynx. Later, however, it undergoes 



