50 A Manual of Veterinary Physiology. 



ment. The ring surrounding the aortic opening in the 

 ox has constantly in its substance one or more pieces of 

 bony tissue ; this is also common in the horse. 



The auriculo-ventricular valves are made up of fibrous 

 and elastic membrane, in which a small proportion of 

 muscular fibre is found close to the attached border. The 

 mitral or bicuspid valve in the horse consists of one large 

 distinct segment, and several smaller ones united to form 

 a second. The tricuspid consists of three segments, one 

 much larger than the others being placed opposite to that 

 portion of the ventricle leading up to the pulmonary 

 artery. 



All the valves are held in position by large and small 

 tendinous cords composed of fibrous tissue, which cords are 

 inserted into large muscular eminences found on the in- 

 ternal surface of the ventricle ; the cords from one eminence 

 do not all pass to one segment of the valve, but to two or 

 three. Their function is to restrain the valves from passing 

 into the auricle during the contraction of the ventricle. 

 Other bands pass from one side of the ventricle to 

 the opposite wall ; they are called moderator bands, and 

 their function is to restrain the ventricular wall from undue 

 dilatation. 



Both the mitral and tricuspid valves meet in the most 

 perfect apposition when the ventricle contracts, and 

 nothing can escape upwards into the auricle. This may 

 be readily demonstrated in the dead heart by tying the 

 aorta and pulmonary veins, and introducing into the left 

 auricle a tube which admits of a powerful jet of water ; the 

 left side of the heart distends and hardens, and at last 

 water forces its way out of the side of the vessel or hole 

 in the auricle in which the tube is inserted. If we now 

 open the auricle, we find the ventricle cut off from view 

 by a tense membranous dome, convex towards the auricle, 

 which is the mitral valve in position ; not a drop of water 

 will escape from the ventricle, though the heart be turned 

 upside down, and it requires some little force to depress 

 the valves. The appearance is a very pretty and singular 



