vi CIRCULATION 89 



its dorsal wall and free ventrally ; and there are also 

 three little scmilunar or watch-pocket shaped valves (v) 

 guarding the aperture between the ventricle and the 

 conus: they are arranged with their edges turned for- 

 wards or towards the cavity of the conus, scr-that any 

 pressure of fluid from the side of the ventricle must 

 force them back and allow ready ingress into the conus, 

 while pressure in the opposite direction must fill them, 

 bringing their edges together, and so barring the 

 passage. 



Anteriorly the longitudinal valve projects in the form 

 of a free flap, and at the same level is a semilunar valve 

 (v r ) : the two together separate the conus proper from a 

 small chamber, the bulbus aortce, from which the right 

 and left carotid (a, a') and systemic (b, b') trunks arise. 

 The pulmo-cutaneous trunks (c) spring from the conus 

 by an aperture (c r ) situated just posteriorly to the valve 

 v' t and itself guarded by a small valve. 



In the dorsal wall of the right auricle is a large trans- 

 verse aperture (s. au. ap). This leads into the sinus 

 venosus : it is therefore called the sinu-auricular aperture ; 

 its two edges are produced into flaps, the sinu- auricular 

 valves, which allow free passage from the sinus to the 

 right auricle, but prevent any flow in the opposite 

 direction. The aperture of the pulmonary veins was 

 referred to on page 85. 



Valves of the Veins. In addition to the valves of the 

 heart, many of the veins contain small watch-pocket 

 valves, all arranged with their concavities directed to- 

 wards the heart, so as to allow of a free passage in that 

 direction. Any attempt to flow in the opposite direction, 

 i.e., from the larger to the smaller veins, will result in 

 filling the valves, bringing their edges into contact 

 with the opposite wall of the vein and thus effectually 

 blocking the passage. 



