784 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



sisting upon this distinction will appear more evident when I come to speak of the 

 influence exerted by these structures on the segments of the bicuspid valve. It is 

 worthy of remark, that while the left apex is closed by two sets of fibres, the left 

 auriculo- ventricular orifice is occluded during the systole by the two flaps or seg- 

 ments constituting the bicuspid valve (Plates XXVIII. and XXIX. figs. 28 and 51 

 m n). The bilateral arrangement, therefore, which obtains in all parts of the ven- 

 tricle and in the musculi papillares, extends also to the segments of the valve in 

 question. What has been said of the arrangement of the fibres in the left ventricle, 

 applies Avith slight modifications to the fibres of the right one ; and many are of 

 opinion (and I also incline to the belief) that the tricuspid valve, is in reality bicus- 

 pid in its nature (Plate XXVIII. fig. 34 m n ; and Plate XXIX. fig. 51 g'h). The 

 shape of the ventricular cavities of the heart of the mammal greatly influences the 

 movements of the mitral and tricuspid valves, by moulding the blood into certain 

 forms, and causing it to act in certain directions. It is seen to advantage when the 

 ventricles are filled with wax or plaster of Paris, and the ventricular parietes re- 

 moved to expose the casts or moulds thus obtained (Plate XXIX. figs. 50 and 51). 

 The form of the left ventricular cavity, which I regard as typical, is that of a 

 cone twisted upon itself (Plate XXIX. fig. 49) ; the twist or spiral running from 

 left to right of the spectator, and being especially well marked towards the apex.* 

 The cone tapers slightly towards its base (h), and the direction of its spiral corre- 

 sponds with the direction of the fibres of the carnese columnar and musculi 

 papillares (Plate XXIX. fig. 50 xi/). As the two spiral musculi papillares project 

 into the ventricular cavity, it follows that between them, two conical-shaped 

 spiral depressions or grooves, are found (Plate XXIX. fig. 49 qj). These grooves, 

 which are especially distinct, are unequal in size ; the smaller one (Plate XXIX. 

 figs. 49 and 50/) beginning at the right side of the apex, and winding in an up- 

 ward spiral direction, to terminate at the base of the external or left and smaller 

 segment of the bicuspid talve (Plate XXIX. figs. 50 and 51 n) ; the larger groove 

 (Plate XXIX. fig. 49 q) beginning at the left side of the apex, and pursuing a 

 similar direction, to terminate at the base of the internal or right and larger seg- 

 ment (Plate XXIX. fig. 51 m). 



Running between the grooves in question, and corresponding to the septal 

 aspect of the ventricular cavity, is yet another groove, larger than either of the 

 others (Plate XXIX. fig. 51 q). The third or remaining groove winds from the 

 interior of the apex posteriorly, and conducts to the aorta {a), which, as the reader 

 is aware, is situated anteriorly. The importance of these grooves physiologically 

 cannot be over-estimated, for I find that in them the blood is arranged or 

 moulded into three sp)iral columns, and that towards the end of the diastole and 

 the beginning of the systole, the blood in the two lesser ones is forced in two 

 spiral streams upon the segments of the bicuspid valve, which are in this 



* In this description tlie Keart is supposed to be placed on its apex. 



