OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 797 



in the second place, that they should descend with the rapidly decreasing columns 

 of blood occasioned by the uniform and continued contraction of the ventricles 

 and musculi papillares, in order that they may adapt themselves to the reduced 

 size of the auriculo- ventricular orifices, and in order that the ventricular cavities 

 may be diminished towards the base, as well as in every other direction. My belief 

 consequently is, that the valves, like the ventricles themselves, have a passive and 

 an active state ; and another which, while it is neither strictly passive nor active, 

 may, for the sake of distinction, be regarded as the neutral state. The passive 

 state, corresponds to the diastole of the ventricles ; the active state, to the systole ; 

 and the neutral or intermediate state, to that brief period which embraces the 

 termination of the diastole and the commencement of the systole.* As, however, 

 the action of the valves, is, to a certain extent, dependent upon, and induced by 

 the action of the ventricles, the following slight differences as regards time, are to 

 be noted. The passive state of the valves corresponds to that period in which 

 their segments are floated meclianically upwards, and their margins partially 

 approximated (Plate XXIX. figs. 52 and 55 mn^ rs) by the blood forced by the 

 auricles into the ventricles, during the dilatation of the latter ; the neutral state, to 

 that almost inappreciable interval which succeeds the sudden contraction of the 

 ventricles, in which the blood set in motion is arranged in spiral columns, and 

 acts in such a way as not only instantly closes the valves (Plate XXIX. figs. 

 53 and 56 mn, rs), but screws and wedges the segments thereof, into each other,} in 

 an upward spiral direction (Plate XXIX. figs. 54 and 57 mn, rs). The active 

 state, corresponds to the period occupied by the progressive contraction of the 

 ventricles. During this period, the valves are dragged forcibly downwards hy the 

 contraction of the musculi papillar^es, in an opposite direction to that hy which they 

 ascended ; and are twisted i7ito or round each other, to form spiral dependent cones. 

 In the active stage, as in the neutral, the blood acts from beneath, and keeps the 

 delicate margins and apices of the segments of the valves, in accurate contact. 

 That the foregoing is the true explanation of the gradual approximation and con- 

 tinued closure of the auriculo-ventricular valves, there can, I think, be little doubt, 

 both from the disposition and structure of the parts, and from experiment. If, 

 e.g., the coagula be carefully removed from perfectly fresh ventricles, and two 

 tubes of appropriate calibre be cautiously introduced past the semilunar valves, 

 and securely fixed in the aorta and pulmonary artery, :j: and the preparation be 



* In speaking of the closure of tlie valves, it is of great importance to remember, that the action, 

 although very rapid, is a strictly progressive one, and necessarily consists of stages. In this, how- 

 ever, as in many other vital acts, it is often very difficult (if not indeed impossible) to say precisely 

 where the one stage terminates and the other begins. 



"I" This act takes place just before the blood finds its way into the aorta and pulmonary artery, 

 the amount of pressure required for shutting and screwing home the auriculo-ventricular valves being 

 less than that required for raising the semilunar ones. 



\ Strictly speaking, the tubes should be introduced into the auriculo-ventricular orifices, as it 

 is through these apertures that the blood passes during the dilatation of the ventricles. As, however^ 



VOL. XXIII. PART III. 10 D 



