Chap. X] 



BLOOD VASCULAR SYSTEM 



167 



cuspid, or mitral valve. It is attached in the same manner as 

 the triscuspid valve, which it closely resembles in structure, except 

 that it is much stronger and thicker in all its parts. 



Function. — These valves oppose no obstacle to the passage of 

 the blood from the auricles into the ventricles because the free 

 edges of the flaps are pointed in the direction of the blood current ; 

 but any flow forced backward gets behind the flaps of the valve 

 (between the flap and the wall of the ventricle), and drives the 

 flaps backward and upward, until, meeting at their edges, they 



OPENING OF RIGHT 

 CORONARY ARTERY 



OPENING or LEFT 

 CORONARY ARTERY 



ANTERIOR. 



SEGMENT CORPUS? 

 ARANTII 



RIGHT 

 POSTERIOR 

 SEGMENT 



LEFT POSTERIOR 

 N SEGMENT 



FIBROUS THICKEN- 

 ING OF EDGE 



Fig. 104. — Aortic Valve. The artery has been cut open between the anterior 

 and left posterior segments, and spread out. (Gerrish.) 



unite and form a complete transverse partition between the ven- 

 tricle and auricle. Being retained by the chordae tendinese, the 

 expanded flaps of the valve resist any pressure of the blood which 

 might otherwise force them back to open into the auricle; the 

 papillary muscles, also, to which the chordae tendineae are attached, 

 contract and shorten at the same time, and thus keep them taut. 

 Semilunar valves. — The valves between the ventricles and 

 arteries are called the semilunar valves (aortic and pulmonary). 

 These valves consist of three half-moon-shaped pockets, each 

 pocket being attached by its convex border to the inside of the 

 artery where it joins the ventricle, while its other border projects 

 into the interior of the vessel. Small nodular bodies, called the 

 corpora Arantii, are attached to the centre of the free edge of each 

 pocket. 



