308 Royal Society. 



from the much greater thickness and density of the lining memhrane 

 of the left auricle. 



The connexions of the anterior mitral valve, being peculiar, re- 

 quire a separate consideration. In dissecting down between the 

 anterior wall of the left auricle and the posterior surface of the 

 aorta, it is seen that the central fibres of the auricular wall are closely 

 attached to the adjacent wall of the vessel. A little further dissec- 

 tion on either side will show that the muscular substance of the left 

 ventricle is deficient between these parts. At the sides indeed it is 

 found, but is gradually lost at some distance from the mesial line. 

 Hence these two orifices (the aortic and left auriculo-ventricular) 

 are not separated as the others are by the intervention of the muscular 

 fibres of the ventricle. The structure and connexions of the ante- 

 rior mitral valve are examined by means of a vertical section, in- 

 cluding the posterior wall of the aorta and the anterior wall of the 

 left auricle. If the lining membrane of the auricle be traced down- 

 wards, it is found to be directly continued on to the posterior surface 

 of the valve, and the membrane on the anterior surface of the valve 

 is continued upwards over the tendinous festooned ring of the aorta, 

 on to the under surface of its semilunar valves. The anterior mitral 

 valve lies beneath a portion of the two posterior arterial valves. 

 The muscular wall of the auricle is observed to terminate by two 

 distinct insertions. The anterior (the larger) division of fibres is 

 attached to the posterior surface of the aorta, opposite to, and below 

 the festooned ring, while the posterior portion is continued directly 

 downwards for a short distance into the valve, and terminates by an 

 attachment to its fibrous tissue. The posterior wall of the aorta 

 descends nearly vertically. Suddenly becoming much thinner oppo- 

 site the upper border of the semilunar valve, it is continued down 

 to the festooned ring, or in other words, it here becomes blended 

 with the base of the semilunar valves. Below this a dense layer of 

 fibrous tissue (which exists below, and fills up the spaces between 

 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, between 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," nearly meeting in the centre, behind, give 

 additional support to the structures entering into the formation of 

 the mitral valve. 



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



