ANIMAL MECHANICS. 



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common tendinous zone which surrounds all the orifices of 

 the heart, with the exception of that of the pulmonary 

 artery. This tendinous zone is shown in Fig. 100.* 



In this figure (AA) is the great central fibro-cartilage, 

 situated upon the septum with which it is incorporated, be- 

 tween the mitral and tricuspid orifices, and behind the right 

 posterior sinus of the aorta. 



The left fibro-cartilage (B) which is much smaller than 

 the central fibro-cartilage, is seated to the left of the mitral 

 orifice, and behind the left posterior sinus, with which it is 

 closely attached. 



At (CC) is shown the strong fibrous loop situated in front 

 and to the left of the root of the aorta, stretching from the 

 left to the central fibro-cartilage, with both of which it is in- 

 timately incorporated. 



At D is shown the tendinous ring of the mitral orifice, 

 stretching from the left to the central fibro-cartilage. 



The tendinous ring of the tricuspid orifice is shown at JE 7 

 attached at both its extremities to the central fibro-cartilage. 



Lastly, at Fis shown the insular fibrous ring that surrounds 

 the orifice of the pulmonary artery ; and is but slightly con- 

 nected with the central mass of fibro-cartilaginous structure. 



All the muscular fibres of the ventricles take origin from 

 the tendinous zone, and having wound round the heart in the 

 manner presently to be described, either return directly to 

 the tendinous zone, or indirectly through the intervention of 

 the columnce carnece and papillary muscles. 



The common fibres of the heart arise from the outer side 

 of the tendinous ring, and from the outer surface of the 

 heart, as shown in Fig. ioi,f which represents the posterior 

 and anterior aspects of the dissected heart. The general 



* Copied by permission from Dr. Sibson's Medical Anatomy. 



f This figure is taken, by permission, from Dr. Sibson's Medical Anatomy. 



2f 2 



