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



sunk in water until the ventricular cavities fill, it will be found, when one of the 

 tubes,* say that fixed in the aorta, is carefully blown into, that the segments of 

 the bicuspid valve roll up from beneath in a spiral direction (Plate XXIX. figs. 

 52 and 55 rs), in a progressive and gradual manner ; each of the two larger or 

 major segments, by folding upon itself, more or less completely, in a direction from 

 within outwards, forming itself into a provisional or temporary cone, the apex of 

 which is directed towards the apex of the left ventricle {^rst stage, in which the 

 crescentic margins and aj)ices of the segments are sloivly approximated hj the uniform 

 expansion of the Hood forced into the ventricle by the auricle). As the pressure 

 exerted by the air is gradually increased, and the action of the valve is further 

 evolved, the segments, folded upon themselves as described, are gradually elevated, 

 until they are on the same plane with the auricido-veniricular fihrous ring ; where 

 they are found to be wedged and screwed into each other, and present a level 

 surface above (Plate XXIX. figs. 53 and 56 rs). 



At this, the second stage of the closure, the crescentic margins of the segments 

 are observed to be accurately applied to each other, to form two perpendicular 

 crescentic walls., which accord in a wonderful manner with similar walls formed 

 by the union of the semilunar valves ; in fact, the manner of closure is, to a 

 certain extent, the same in both ; the segments in either case, being folded upon 

 themselves by the blood, and presenting delicate crescentic margins, which are 

 flattened against each other, in proportion to the amount of pressure employed. 

 When the crescentic margins of the segments, are so accurately applied to each 

 other as to become perfectly unyielding, and the distending process is carried 

 beyond a certain point, the bodies or central portions of the segments bulge in an 

 upward or downward direction as happens ; the segments of the mitral valve, 

 protruding into the auricle (Plate XXIX. figs. 54 and 57 rs) ; those of the semi- 

 lunar one, into the ventricle (Plate XXVIII. figs. 27 and 28 vmx). 



This completes the first and second stages of the process, by which the mitral 

 or bicuspid valve is closed ; but the more important, as being the more active 

 and difficult, is yet to come. This consists in adapting the segments of the valve., to 

 the gradually diminishing auriculo- ventricular orifice ; and in dragging them down into 

 the left ventricular cavity, to diminish the ventricle towards the base. By this 

 act, the segments, as has been shown, are made to form a spiral dependent cone, 

 an arrangement which renders the obliteration of the ventricular cavity towards 

 the base, a matter of certainty. 



The third stage of the closure of the mitral valve, entirely differs from the 



the insertion of tubes, however small, into the anriculo-ventricular openings would necessarily prevent 

 the complete closure of the valves, there is no good reason why the plan recommended in the text 

 should not be adopted. 



* Thin metallic tubes with unyielding parietes are best adapted for this purpose, as they can 

 be readily fixed in the vessels, and the amount of pressure exercised by the breath on the valves 

 asily ascertained. 



