148 



tiaily. The act of contraction of the vein is manifested b}'' pro- 

 gressive constriction of its calibre and increasing thickness of its 

 wall ; the relaxation of the vessel, by a return to the former width 

 of calibre and thickness of wall. The rythmical contractions and 

 dilatations of the veins are continually going on, and that, on an 

 average, at the rate of ten contractions in the minute. The con- 

 tractions centrad and distad of a valve appear to be simultaneous, as 

 also the dilatations. 



During contraction, the flow of blood in the vein is accelerated, 

 and on the cessation of the contraction, the flow is checked, with a 

 tendency to regurgitation, which brings the valves into play. But 

 this check to the onward flow of the blood is usually only momen- 

 tary ; already, even while the vein is in the act of again becoming di- 

 lated, the onward flow recommences and goes on, though with com- 

 parative slowness, until the vein contracts again. It is the heart's 

 action which maintains the onward flow of blood during the dilata- 

 tion of the vein, whilst it is the contraction of the vein, coming in 

 aid of the heart's action, which causes the acceleration. 



The valves are composed sometimes of but a single flap, some- 

 times of two. In the situation of a valve and centrad of the inser- 

 tion of its flaps, the veins present the usual dilatations or sinuses. 

 The valves are a reduplication of the clear innermost coat of the 

 vein, with sometimes an intervening layer of cellular tissue. 



The veins closely accompany the arteries, the nerve only inter- 

 vening. 



The contractility of the arteries the author finds altogether difl^'er- 

 ent from that of the veins, being tonic, not rytlimical. He has not 

 been able to observe unequivocal evidences of tonic contractility of 

 the veins, which they have been alleged to possess. 



In figure 3, of drawing No. 1, illustrating his paper, the author 

 represents, in reference to this point, an artery and a vein, as ob- 

 served imm^ediately after pressure had been applied over them. The 

 artery is seen constricted at intervals both above and below the place 

 of pressure. The vein is not so constricted, but at the place where 

 the pressure was applied there is seen a greyish granular dejDosit 

 of lymph within the vessel, giving rise to an appearance of constric- 

 tion by narrowing the stream of blood. On watching a vein in this 

 state, the author has observed portions of the lymphy deposit car- 

 ried away by the stream of blood, with corresponding enlargement 

 of the channel. 



The author further finds that nowhere do the arteries and veins 

 of the web of the bat's wing directly comm.unicate, as has also been 

 alleged ; the only communication being the usual one through the 

 medium of capillaries. 



In an appendix to this paper, the author describes the result of 

 his microscopical examination of the structure of the veins and arte- 

 ries. Both artery and vein have a middle coat of circularly disposed 

 muscular fibres ; but the appearance of the fibres is diff'erent in the 

 two vessels. The fibres of the vein are g-gL-dth in. broad, pale, 

 grayish, semitransparent and granular looking. In general aspect. 



