ANATOMY. 



3. Arteries in general do not pursue a 

 straight, but 'his is 



as in the spermatics, thpWS of tin- fare and 

 occiput, and in most of the smaller arte- 

 ries. 



4. Though the ramification of arteries 

 maybe compared to the branching 1 of 



ct it differs materially in this par- 

 ticular, that the different branches fre- 

 qtieiith conjoin. This conjunction is tech- 

 nically tcrmedjif we borrow the term from 

 nguagc, their anastomosis ; if 

 from tile Latin, their iwixculuti-ni. This 

 union of arteries rarely happens among 

 the larger ones, but frequently amongthe 

 smaller ; and increases in number in pro- 

 portion to the minuteness of the vessels. 

 The utility of the inosculations of arteries 

 is evident; were it not for this circum- 

 stance, if any arterial trunk were acci- 

 dentally compressed, so that the current 

 of blood in it should be for some time 

 obstructed, the parts which it supplied 

 must perish. But in consequence of the 

 frequent communications of these tubes 

 with each other, the blood can pass from 

 the adjacent arteries into all the branches 



.v one accidentally obstructed. 

 When arteries inosculate, two currents 

 of blood, moving in opposite directions, 

 must come together, and retard each 

 other's motion. This probably is the rea- 

 son, why larger arteries, in which the 

 blood flows with rapidity, so seldom con- 

 join ; whilst the smaller ones, in which 

 the blood's motion is more tardy, commu- 

 nicate in surprising numbers, and with a 

 frequency proportionate to their minute- 

 ness. The very frequent communications 

 of the minute arteries prevent the preju- 

 dicial consequences of obstruction of t he- 

 trunks almost as effectually, as if those ar- 

 teries themselves communicated by more 

 direct and larger char .: 



All these minute arterial tubes are capa- 

 ble of enlargement ; and it is an ascertain- 

 ed fact, that even the aorta itself may be 

 s^radually obstructed at some distance from 

 the heart, without the parts which it sup- 

 plies being deprived of nourishment. 

 From an attentive consideration of all 

 these circumstances, it lias been conclud- 

 ed, that the mode nit e increase of tin- 

 , the branches oflarp- arteries; tin- 

 acute angles at which they divide ; their 

 early rectilinear course ; and the rare oc- 

 currence of inosculation between them; 

 are designed to facilitate the rapid motion 

 of the blood in them, so that it may arrive 

 unchanged, and in the same state that it 

 was in when projected from the heart, at 

 that part of the body, for the nourishment 



of which it was intended : whilst, on the 

 contrary, the K^ at increase of the . 



.the varii'\ ofi. 

 rtuous course, and their fre- 

 quent communications, were designed to 

 cheek the velocity of the blood's motion, 

 when it has arrived at that part, where 

 ti is to be performed, and nutrition 

 is to take place. Contrary opinions have 

 indeed been maintained; and for the fur- 

 ther discussion of this subject, we must. 

 refer the reader to the remarks on the 

 circtiliitioH in the article PHTSIOI.DGT. 



Termination of the arteries. When these 

 vessels have become very minute, they 

 terminate in two ways: they either turn 

 bark again, and become veins, and return 

 the blood to the heart, or they send oft' 

 fine vessels, which abstract something 

 from the circulating blood, and are there- 

 tor., called secerning arteries. Though 

 none but minute arteries are ever reflect- 

 ed to become veins, yet many of them arc 

 of sufficient magnitude to admit common 

 waxen injection ; and when this experi- 

 ment succeeds, the continuity of the arte* 

 ries and veins is very manifest It seems 

 therefore to follow from this facility .t 

 communication, that the mass of the blood 

 is constantly and freely circulating, in or- 

 der to undergo that change which is ef- 

 fected in the lungs, whilst but a small part 

 of it proceeds into the very minute arte- 

 ries, for the purpose of having secretions 

 made from it. For these arteries, In 

 minute, must be considered large, in com- 

 parison with the exility of others, which 

 cannot be injected with wax, ;u 

 reject the- red globules of the blood, or 

 admit them in such small proportion, that 

 they do not impart the red colour to the 

 fluid which moves in those \ 

 \ve may venture to affirm that th< 

 bules do not much exceed in diameter 

 the 150,000th part of an inch, which cir- 

 cumstance sufhVii-ntly shows the minute- 

 lesser arteries. 



The se'-ernin^ arteries are in general 

 too minute to admit . : ration; 



they are hov, 



in the kidney, for -nay be 



seen cont'm 



Subtile injections, wh. nto the 



larger arterial trunks, ooze out on t 



s, and int' 



sul;st:ii ''are generally supposed 



to be |> orifices 



reforc, 



rathe; ' nstru'.ion, . 



to belicv <.', that the secerning arteries ab- 

 stract the particles of nutrition, or thcma- 

 terials which compose the fabric of the 

 body, from the circulating fluids, and de- 



