137 



the attached bases of the semilunar valves) descends for some di- 

 stance into the anterior mitral valve, immediately behind its anterior 

 surface. It is by a close attachment to the posterior surface of this 

 layer that the muscular fibres of the auricular wall which descend 

 into the valve, terminate. This layer of fibrous tissue, however, 

 may be generally traced downwards into the valve farther than the 

 muscular fibres. 



The boundary, then, betv/een the aortic and auricular apertures 

 is formed above the mitral valve by the posterior wall of the aorta, 

 terminating at its junction with the bases of the semilunar valves, 

 and immediately below the posterior surface of which is attached 

 the greater portion of the muscular fibres forming the anterior wall 

 of the left auricle. The extremities of the two bones which in ru- 

 minants replace a portion of the lateral and posterior divisions of 

 the " festooned ring," nearlj'- meeting in the centre, behind, give 

 additional support to the structures entering into the formation of 

 the mitral valve. 



In examining the structure and connections of the auriculo-ventri- 

 cular valves, it is noticed that a considerable portion of tendinous 

 fibres pass from the insertions of the cords, through the valves, to 

 the zones, and many of the smaller cords pass up directly into the 

 angle formed between the under surface of the valve and the inner 

 surface of the ventricle, and at once enter into the formations of the 

 fibrous zones. These cords are short, and many of them spring from 

 the wall of the ventricle, behind the valve. Therefore it results, 

 that these zones are densest and most strongly marked in those 

 portions corresponding to the attached borders of the valves, and 

 gradually become less distinct towards the intervals between them. 

 Hence the greater portion of the auriculo-ventricular zones is more 

 properly to be considered in connection with the valves. 



The fibres of elastic tissue exist in the auriculo-ventricular valves, 

 but more sparingly than in the arterial valves. 



The many contradictory statements which have been advanced 

 concerning the existence of muscular fibres in the auriculo-ventri- 

 cular valves, may perhaps be explained by a consideration of the 

 mode in which the muscular fibres of the auricles terminate, which 

 has been already described. The internal fibres which have been 

 mentioned, descending from the auricular walls into the valves just 

 beyond their attached margins, may be traced to a greater distance 

 in some cases than in others. They generally terminate by a tole- 

 rably well-defined margin, but this varies. They usually descend 

 for a greater distance between the layers of the anterior mitral valve, 

 immediately beneath its auricular surface ; but even here they are 

 seldom found stretching far into the valve, not terminating, however, 

 so abruptly. 



Therefore, if a portion of the attached border of a valve imme- 

 diately below its upper surface be examined, muscular fibres in 

 abundance will generally be detected ; w^hereas if sought for in any 

 other portion of the valve far from its attached border, according to 

 the foregoing observations, they will not be found. 



