The Anatomy 0/ /« H O R S E. 105 



virtue of the fpiral Diredion of its Fibres, it is thereby en- 

 abled to bear the frequent Sallies of the Blood in its Expul- 

 fion from tbe Heart ; and left thefe Fibres fliould feparate 

 upon any violent Impulfe, the innermoft Coat, though a 

 fine tranfparent Membrane, yet it is wove fo clofe, as to 

 be able to preferve the middkmolt, and keep the Blood 

 wilhin its proper Channels. 



It is moreover to be obfervcd, as the Arteries are conical 

 Channels, and grow gradually fmaller, fo their Coats grow 

 proportionably thinner. And the Coats of q-jygQ ^^ f.L 

 the V'^eins feem, according to the Opinion of Veins aContU 

 the moft modern Anatomifts, to be only a nuation ofthoft- 

 Continuation of the Coats of the capillary of the Arteries. 

 Arteries, reflected back again towards the 

 Heart. But ahhough the Coats of the Veins be the fame 

 with thofe of the Arteries, yet it is to be taken Notice of, 

 that the mufcular Coats of all the Veins are as thin as in 

 the capillary Arteries ; the Preflure of the Blood againft: 

 the Sides of the Veins being much weaker than that againft 

 the Sides of the Arteries, and therefore not requiring its 

 Channels to be fo thick and ftrong. 



The Veins are not endu'd with Pulfation, as the Arteries, 

 becaufe the Blood falls into them with a continual Stream 

 from the capillary Arteries, which by reafon of their Small- 

 nefs, have only a very weak, or fcarcely any Motion ; and 

 then as it advances towards the Heart, it moves from a 

 narrow Channel to a wider ; and therefore its Motion 

 would have been extremely languid and flow, had not Na- 

 ture contriv'd feveral Helps to promote its <rhe Feins have 

 paflage. For that Reafon, as it is the Of. halves, 

 iice of the Veins to return and carry back 

 all the Blood to the Heart, there is to be feen in moll of 

 them (efpecially in fuch as have their Direction upwards) 

 feveral Valves at convenient Diftances, fometimes one, and 

 fometimes more, like fo many half Thimbles ftuck to their 

 Side, with their Mouths towards the Heart ; and as the 

 Blood moves that Way, they are prefled clofe to the Sides 

 of the Veins ; but if it (hould fall back, it muft fill the 

 Valves, and ftop up the Channel, that no Blood can repafs 

 them. And beiides thefe Valves, it is alfo obfervable, that 

 in many Places where there is a Branch of a Vein, there is 

 an Artery lies under it, which by its continual Pulfations, 

 helps to forward the venal Blood towards the Heart ; fo 

 fhat albeit the Blood moves from a narrow Channel into a 



widerj 



