130 



THE ANATOMY OF THE HORSE. 



papillares, each with its set of chordae tendineae, and three large cusps ; 

 but it will be observed that one set of the chordae tendineae does not 

 pass entirely to one cusp, but divides itself between two adjacent 

 segments. » • 



Directions. — A better view of the tricuspid valve may now be obtained 

 by cutting through the auriculo-ventricular ring near the point where 

 the first incision was begun in opening the ventricle, selecting the 

 interval between two cusps. When some of the chorda: tendineae have 

 been cut, this will enable the wall of the ventricle to be thrown 

 outwards. 



When the ventricle contracts, the blood, prevented by the tricuspid 

 valve from passing back into the auricle, is forced upwards into the left- 

 anterior portion of the ventricle, and 

 leaves the cavity by the pulmonary 

 artery. This portion of the cavity 

 which leads up to the artery is termed 

 the conus arteriosus. The orifice of the 

 pulmonary artery is surrounded by a 

 valve composed of three crescentic seg- 

 ments, and hence termed the semilunar 

 valve. The convex border of each seg- 

 ment is fixed to the wall of the artery 

 where it springs from the ventricle. 

 The concave border is free, and shows 

 at its mid point a minute, fibro-cartila- 

 ginous thickening — the nodulus or corpus 

 Arantii. On each side of the corpus a 

 small crescentic portion near the free 

 edge of the segment, and distinguished 

 from the rest by its thinness, is termed 

 the lunula. One surface of the valve 

 is convex, and, during contraction of the 

 ventricle, it is directed to the blood- 

 stream ; the other is concave, and 

 directed to the wall of the artery, which, 

 opposite each segment, forms a pouch — 

 the sinus of Valsalva. When the 

 ventricle has ceased to contract the elastic recoil of the artery forces 

 the blood against the concave side of the segments, and carries them 

 inwards till they meet and completely close the opening. The blood is 

 thus propelled along the pulmonary arteries to the lungs, where, in the 

 capillary plexus on the walls of the air-cells, it is purified. The 

 purified fluid is then carried from the lungs by the pulmonary veins, 

 which pour it into the left auricle of the heart. 



Fig. 11. 



Diagram of the two Cavities of 



THE LEFT SIDE OF THE HEART (Ellis). 



le. Left pulmonary veins ; i. Right 

 pulmonary veins ; o. Remains of fora- 

 men ovale; I. Left auriculo-ventri- 

 cular opening ; m. Auricular appendix ; 

 n. Aperture of the aorta. 



