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ries is often tortuous, and it is observed, that the arteries which are sent 

 to hollow viscera, as the stomach, the uterus, and the bladder, or other 

 parts capable of contracting, of stretching, and of changing their dimen- 

 sions every moment, as the lips, are much the most curved, no doubt, 

 that they may by unfolding, give way to the extension of the tissues into 

 which they are distributed. Lastly, the arteries arise from one another, 

 and form with the trunk or branch from which they are given off, an 

 angle varying in size, but which is always obtuse, and more or less acute 

 towards the branch. 



As the arteries recede from their origin, they communicate together, 

 and these anastomoses form arches, two branches bending towards each 

 other, and joining at their extremities, as we see in the vessels of the 

 mesentery; sometimes two parallel branches meet at an acute angle, 

 and unite into one trunk, thus the two vertebrals join to form the basilar 

 artery: some communicate by transverse branches, which pass from the 

 one to the other, as is seen within the skull. 



In the anastomoses of the first kind, the columns of blood Sowing, in 

 contrary directions, along the two branches, meet at the point of union? 

 and mutually repel each other, their particles mingle, and lose much of 

 their motion in that reciprocal shock. The blood then follows a middle 

 direction, and enters the branches which arise from the convexity of 

 these anastomotic arches. 



When two branches unite to produce a new artery, of a greater calibre 

 than each taken separately, but not so large as both together, the motion 

 of the blood becomes accelerated, because it passes from a more capacious 

 into a straighter channel, and the forces which determined its progression, 

 are concentrated into one. Lastly, the transverse anastomoses are well 

 calculated to promote the passage of the blood from the one branch into 

 the other, and to prevent congestion in the parts. 



LV. The arteries are imbedded in a certain quantity of cellular tissue, 

 are almost universally accompanied by corresponding veins, by lympha- 

 tics and nerves, and their coats are thicker in proportion as their calibre 

 is smaller. The experiments of Clifton Wintringham prove, that the 

 parietes are stronger in the small than in the large arteries, hence it is 

 observed, that aneurisms are much less frequent in the former. Their 

 parietes have sufficient firmness not to collapse, when the tube of the ar- 

 tery is empty. They are formed of three coats; the external or cellular 

 admits of considerable extension, and appears to be formed by the con- 

 densation of the lamina of the cellular tissue, which surrounds the arte- 

 ry, and unites it to the neighbouring parts. The second coat is thicker 

 and firmer, of a yellow colour, and fibrous, and is by some considered as 

 muscular* and contractile, while other physiologists merely allow it to 

 possess a considerable degree of elasticity. The longitudinal fibres, ad- 

 mitted by some authors in the texture of this second coat cannot be dis- 

 tinguished, and their existence is not necessary to account for the longi- 

 tudinal retraction of arteries. In fact, this retraction might depend on 



* If in man and the greater number of animals, the yellow fibres which form this 

 coat, differ greatly from muscular fibres, they in the elephant resemble that texture 

 very completely, as I had an opportunity of observing 1 , when I witnessed the dissection 

 of the elephant that died in the year 1802, at the Museum of Natural History. Let men 

 of judgment decide whether the analogy is sufficient to warrant our admitting, in the 

 arteries of the human body, the existence of muscular fibres. Author's Nate. 



