284 CIRCULATORY APPARATUS. 



belong to two distinct types, that of the sigmoid and that of the 

 cuspid valves. This difference of type may be dismissed in a 

 few words. The sigmoid valves are dnplicatures of the intima, 

 each of which forms a semicircular fold with its convexity 

 directed towards the current; the terminal points of the lines 

 along which the valves are attached, being all in the same hori- 

 zontal plane. Accordingly they form pouches, which touch one 

 another by their outer surfaces when they are filled with blood, 

 and so close up the tube. The pecidiar way in Avhicli these flaps 

 are attached, renders it impossible for them to move otherwise 

 than ivith the current. The cuspid valves on the other hand, ' 

 consist of moveable curtains, attached in the same horizontal 

 plane, at right angles to the axis of the vessel; and these, so far 

 as their mode of attachment is concerned, might be folded over to 

 either side indifferently ; nevertheless, they too can only yield 

 to the blood-current in one direction, inasmuch as the union of 

 their free borders with the chordas tendinece prevents their being 

 forced into the cavity of the auricle. 



It would indeed be matter for surprise had Natm-e, usually so 

 simple in the methods she adopts, really contrived two kinds of 

 valve in the present case, diff'ering from one another in principle ; 

 we ought therefore to consider whether the apparent diversity of 

 anatomical construction may not be reducible to a simple modi- 

 fication of one and the same fundamental ty}3e. Now it seems 

 to me that this type is to be found in the sigmoid form of valve ; 

 not only because of its more frequent occurrence in the organism 

 (valves of veins, &c.), but also because the innocuous variety of 

 perforation which Ave are now considering, presents us with a 

 modification of the sigmoid valve, tending to approximate it to 

 the cuspid type. 



This perforation, as may be seen at a glance from the 

 annexed figure, may be regarded as a partial separation of the 

 free border of the valve, extending from the nodulus Arantii to 

 its terminal attachment. Now during the closure of the valves, 

 this free border acts as a retinaculum ; it hinders the valve from 

 receding to too great an extent. One need only put the tip of 

 one's forefinger into a sinus of Valsalva, to be convinced 

 that the valvular pouch is narrower at its entrance than at 

 the ^'line of contact;" i.e. that the free margin of the valve 

 forms a shorter and straighter bond between the nodulus Arantii 



