34 



A MANUAL OF VETERINARY PHYSIOLOGY 



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firm attachment. 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. 



Valves of the Heart. — The auriculo-ventricular valves are made 

 up of fibrous 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 which leads 



to the pulmonary 

 artery. 



The free edges of all 

 the valves are held in 

 position by large and 

 small tendinous cords 

 (chordce tendinece) com- 

 posed of fibrous tissue, 

 which are inserted into 

 musculi papillar es found 

 on the internal surface 

 of the ventricle ; the 

 cords from one papilla 

 do not all pass to one 

 segment of the valve, 

 but to two or three 

 (Fig. 14). The function 

 of the papillae is to 

 restrain the valves from 

 being forced too far 

 into the auricle during 

 the contraction of the 

 ventricle, and this they 

 accomplish by gradually . 

 shortening as the walls of the ventricle approximate ; compen- 

 sating by their shortening for the movement of the ventricular 

 wall, and thus exerting traction on the cords (Fig. 15). This 

 shortening is brought about by the layer of muscular fibres which 

 at the apex passes from the external face of the heart into the 

 interior of the ventricles, and thence to the fleshy columns and 

 papillary muscles (p. 32). Other bands pass from one side of the 

 ventricle to the opposite wall ; they are called moderator bands, 

 and their function is to protect the ventricular wall from undue 

 distension. 



The valvular flaps meet in the most perfect apposition when 

 the ventricles contract ; their edges are inverted, and the sides of 



Fig. 14. — Left Ventricle of Horse exposed 

 to show Mitral Valve. 



1, Portion of valve ; 2, columnce carnece, on the 

 upper surface of which are found the musculi 

 papillares, to which the chordce tendinece are 

 attached. 



