312 CIRCULATION OF BLOOD. LECT. XVII. 



of three portions of membranes, which have, as their name 

 indicates, a semi-circular form ; on one side they are 

 adherent to the wall of the orifice, and on the other are free, 

 so that when they are depressed, they present the form of 

 three triangles, having for a common summit the centre of 

 the vessel, and for a base, its circumference. They are 

 pressed against the sides of the vessel, when a liquid column 

 is driven out of the heart ; but, on the contrary, when a 

 liquid attempts to pass back from the vessel to the heart, 

 they immediately expand, and close the orifice. Thus then 

 they allow the blood contained in the heart to escape by 

 the vessels, and prevent the blood contained in the vessels 

 from returning into the heart. 



Blood-vessels. Lastly, we shall say a few words respect- 

 ing the blood-vessels. In the arteries, we remark a mid- 

 dle membrane, very thick, and composed of circular fibres 

 of a peculiar nature, called the yellow elastic tissue. To 

 this the arteries owe their elasticity. The membrane, 

 composed of this tissue, is placed between the internal or 

 serous membrane and the external membrane, composed 

 of condensed cellular tissue. Three tissues likewise form 

 the sides or walls of the veins ; but in these vessels the 

 yellow elastic tissue is less obvious ; the internal membrane 

 is enveloped by a thin layer of slight longitudinal fibres, of 

 a loose texture ; and the external membrane is of a cellu- 

 lar nature, and very resisting. The internal coat is thin, 

 smooth, resisting, and extensible. It forms in the cavities of 

 some of the venous trunks numerous folds or valves, which 

 are so placed that they can diminish or even entirely ob- 

 literate the diameter of the vessel, when a liquid column 

 moves in a direction contrary to that of the course of the 

 blood within them. 



The Circulation. In a few words we shall describe the 

 manner in which the circulation is effected. To give you 



