OF THE VALVES OF THE VASCULAR SYSTEM IN VEIITEBRATA. 785 



way progressively elevated towards the base, and twisted and wedged into each 

 other, until regurgitation is rendered impossible (Plate XXIX. fig. 51 m n). When 

 the bicuspid valve is fairly closed, the blood is directed towards the third 

 and largest groove, which, as has been stated, communicates with the aorta. The 

 spiral action of the mitral valve, and the spiral motion communicated to the blood 

 when projected from the heart, is due to the spiral arrangement of the musculi 

 papillares and fibres composing the ventricle, as well as to the spiral shape of the 

 left ventricular cavity. These points are determined in the following manner : — 

 When a cast of the interior of the left ventricle is made, by introducing liquid 

 plaster of Paris into the left ventricular cavity, by means of a tube inserted 

 into the aorta and reaching to the left apex, it is found, on cutting away the 

 parietes of the ventricle, that the segments of the mitral valve are borne up on 

 the plaster,* and wedged into each other on a level with the ventricular orifice. 

 It is further found, that the two spiral streams of plaster (now spiral columns) 

 which closed the segments of the mitral valve, merge towards the base, into 

 the third column, communicating with the aorta. That portion, therefore, of the 

 left ventricular cavity (Plate XXIX. figs. 50 and 51 o), which corresponds to the 

 conus arteriosus or infundibulum of the right one (Plate XXIX. fig. 50 ^), is 

 conical in form. It is moreover furnished with three conical-shaped spiral 

 depressions, which in the cast appear as conical-shaped sjnral prominences {p 6), 

 and are continuous with the three spiral columns of plaster proceeding from the 

 apex of the ventricle. As the apices of the three conical-shaped infundibuliform 

 prominences referred to are directed between the three segments of the aortic 

 semilunar valve, the blood from this arrangement must on its onward progress 

 throw the semilunar segments hastily apart, by causing them to fall back upon the 

 spirally disposed sinuses of Valsalva (^w^). The spiral channel, which is thus 

 provided for the blood, is not confined to the heart, but extends for a short distance 

 into the great vessels. As the semilunar valves are closed by a reverse move- 

 ment to that by which they are opened, it is not difficult to perceive how the 

 spiral action of the segments constituting them is induced. 



What has been said of the left ventricular cavity and aorta, applies, with slight 

 alterations, to the right ventricular one and the pulmonary artery (Plate XXIX. 

 figs, 50 and 51 cz), the cone formed by the right cavity being flattened out and 

 applied to or round the left. 



Intricate Structure of the Mitral and Tricuspid Valves in Mammalia ; Eelations of 

 the cord^ tendine^ to the segments and to the musculi papillares. 



The auriculo- ventricular valves are composed of segments, which differ in 

 size, and are more or less triangular in shape. They are much stronger than 



* In order to see the spiral movement of the segments to advantage, the plaster ought to he , 

 made very thin. Should any difficulty occur, the experimenter is recommended to use water until 

 he is familiar with the phenomena to be obsei'ved. 



VOL. XXIIL PART III. ] A 



