ART 



generally called circumflex. Sometiiiies, indeed, a l^.rge 

 'artery does proceed from the trunli at a greater angle, 

 nearly a right angle, as the renal arteries, &c. Though the 

 large' arteries generally ramify at acute angles, there is great 

 divtrfity in the bnmching of the fmaller ones. 



3. Arteries, in general, do not purfue a ftraight, but 

 fer^)entinc courfc ; in fomc inllances it is remarkably tiie 

 cafe; a$ in the fpermatics, ihofe of the face and occiput, 

 and in moll of the fmaller arteries. 



4. Though the ramification of arteries may be compared 

 to the branching of trees, yet it differs materially in this 

 particular, that the different branches frequently conjoin. 

 This conjundion is technically termed, if we borrow the 

 phrafc from the Greek language, their " Anaflomofis ;" 

 if from tlie Latin, their " Inofculation." This union of 

 arteries rarely happens among the larger ones, but fre- 

 quently am6ng the fmaller ; and iiicreafes in number in 

 proportion to the minuteiicfs of tlie veffels. The utility of 

 the inofculation of arteries is evident : were it not for this 

 circumllance, if any artcrious trunk were accidentally cora- 

 prcfTed, fo that the current of blood in it fliould be for fome 

 lime obllruAed, the parti which it fupplied mull pcrilh. 

 But in confequence of the frequent communication of the 

 arteries with one another, the blood can pafs from the 

 adjacent arteries into all the branches of any one accident- 

 ally obllru£led. 



When arteries inofculate, two currents of blood, moving 

 in oppofite direftions, mull come together, and retard each 

 other's motion. This probably is the reafon that larger 

 arteries, through which it feems necetfary that the blood 

 (hould flow with rapidity, fo feldom conjoin, whilll the 

 fmall arteries, in which it is requifite the blood Ihould move 

 tardily, communicate in furprifmg numbers, and with a 

 frequency proportionate to their minutenefs. The very 

 frequent communication of the minute arteries, almoft as 

 effcdually prevents the prejudicial confequences of obftruc- 

 tion in the larger trunks, as if thofe arteries tiiemfelves were 

 made to communicate by more direft and larger channels. 

 AU thcfe minute arterious tubes are capable of enlargement, 

 and it is an afcertained faft, that even the aorta itfelf may 

 be gradually obftrufted, without the parts which it fupplies 

 being deprived of nourifhment. From an attentive conlider- 

 ation of all thefe circumflances, it has been concluded, that 

 the moderate increafe of the area of the branches of large 

 arteries, the acute angles at which they divide, their nearly 

 reClilinear courfc, and the rare occurrence of inofculation 

 between them, are defigned to facilitate the rapid motion of 

 the blood in them, fo that it may ai-rive unchanged and in 

 the fame flate that it was projected from the heart, at that 

 part of the body for the nourilliment of which it is in- 

 tended; whilll, on the contrar)-, the great increafe of the 

 area of the fmaller ve3els,the variety of tlieir angles, theirtor- 

 tuous courfe, and their frequent communications, were 

 defigned to check the velocity of the blood's motion, when 

 it has arrived at that part where fccretion is to be performed, 

 and nutrition is to take place. Contrary opinions have indeed 

 been maintained ; and for the further difcuffion of this fubjeft, 

 we mufl refer the reader to the Circulation of the Blood. 



Arteries, Termination of. When tlie arteries have be- 

 come very minute, they terminate in two ways; they either 

 turn back again and become veins, and return the blood to 

 the heart, or they fend off fine vefTcls which abflraft fome- 

 tiiing from the circulating blood, and which are therefore 

 called the fecerning arteries. Though none but minute ar- 

 teries are ever reftefted fo as to become veins, yet many of 

 them are of fufBcient magnitude to allow the palfage of 

 contmoa waxen injection. The arrangement of the zniuute 



ART 



veins can he demondrated by impelling common waxen in« 

 jection into the arteries, particularly if a de^'^^e ui putrefac- 

 tion be fufl'ered to take place pievioufly to the experiment. 

 In the difTeftion of fuch a preparation, the continuity of the 

 arteries and veins is very manifcll. It feems therefore to 

 follow from this facility of communication, that the mafs of 

 blood is conllantly and freely circulating, in order to under- 

 go that change which is effecled in the lungs, whilll but a 

 fmall part of it proceeds into the vtvy minute arteries, for 

 the purpofe of having I'rcretions m.ade from it. For thcfe 

 arteries, however minute, nuiil be coiilideicd large in compa- 

 rifon to t)ie exility of olheis, which cannot be injedlcd with 

 wax and even rejeft ihc red globules of the blood, or admit 

 them in fuch Imall proportion, that they do not impart the 

 red colour to the fluid which moves in thefe vcfTcls. Now 

 we may venture to aflirm, that thefe globules do not much 

 exceed, in diameter, the ijO.ocoth part of an inch, which 

 circumllance fufficiently fhcws the minutenefs of the Icffer 

 arteries. See the article Blood. But however minute arte- 

 ries may become, ftill thty mull all end in the fame manner ; 

 they mull be continued into veiri:;, for that is the route which 

 the blood, or fubtile injedlions purfue, and from the molt 

 minute arteries thole which perform fccretion arife. 



The fecerning arteries are too minute to admit commonly 

 of demonllration ; they are however evident in fome glands ; 

 in the kidney for iiillance, they may be fecn continued into 

 the excretory veffels or tubuli uriniferi. Subtile injedlions, 

 when thrown into the larger arterial trunks, may be feen 

 oozing out on the iurfaces of m.embranes, and into the cel- 

 lular fubllnnce, and they are generally fuppofed to be poured 

 forth from the open orifices of the fecerning arteries. Ana- 

 logy therefore, rather than aftual demonllration, leads us to 

 believe, that the fecerning arteries abllrad the particles of 

 nutrition, or the materials which compofe the fabric of the 

 body, from the circulating fluids, and dcpolit them from their 

 open mouths, fo as by this means to build up and keep ia 

 repair the ftrudlure of the body. 



Arteries, Dijlribution of. The great artei-y, whofe 

 branches fiipply the whole of the body, is named the " aor- 

 ta." It comes oil from the upper and back part of the left 

 ventricle, where it is furroundcd for a fhort part of its courfe 

 by the flefhy fibres of the heart. Its origin appears externally 

 to be divided into three diftinft eminences, which denote the 

 fituation of its femilunar valves. 



The aorta emerges from the bafis of the heart, between 

 the pulmonaty arter)', and the right auricle. It afcends at 

 firft rather to the right, till it arrives at the upper edge of 

 the fecond rib. Then it begins to bend backwards acrofs 

 the divifion of the pulmonary artery and of the trachea, till it 

 reaches the left fide of the fpine, in which fituation it de- 

 fcends from the fourth or fifth dorfal to the lall lumbar ver- 

 tebra. 



By the " arch of the aorta," is meant that part of the 

 vefiei which arifes from the heart, and bends acrofs the chcfl. 

 It fends off the following branches : viz. the two coronary 

 arteries, whofe mouths are fituated jull above the upper edge 

 of the femilunar valves. They depart from the trunk at 

 right angles, and are diHributed to the lieart illclf. The 

 moil convex part of the arch fends off three large branches ; 

 firft, the artcria innominata ; fecondly, the left carotid arte- 

 ry ; and thirdly, the left fubclavian artery. Varieties not 

 unfrequently occur in the number of arteries which arife 

 from this upper part of the arch : a long lift of them may 

 be fecn in Soemmerring de coi-poris humani fabrica, torn. v. 

 p. 120. 



The right coronary pafTes in the groove between the 

 right auricle and ventricle, covered by fat, to the flat fur- 

 face 



