VI CIRCULATION 89 



conus, so 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') : 

 the two together separate the conus proper from a small 

 chamber, the bulbus aorta, from which the right and left 

 carotid (a, a) and systemic {b, V) trunks arise. The pulmo- 

 cutaneous trunks (c) spring from the conus by an aperture 

 {(!) situated just posteriorly to the valve v', and itself 

 guarded by a small valve. 



In the dorsal wall of the right auricle is a large transverse 

 aperture (s. au. afi). 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. 



Valves of the Veins. — In addition to the valvee of the 

 heart, many of the veins contain small watch-pocket valves, 

 all arranged with their concavities directed towards 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 filhng the valves, bringing 

 their edges into contact with the opposite wall of the vein, 

 and thus effectually blocking the passage. 



Circulation of the Blood. — We see then that an inves- 

 tigation of the structure of the heart shows that fluid can 

 traverse it in one direction only, viz., from the sinus to the 

 right auricles, from the auricle to the ventricle, from the 

 ventricle to the conus, and from the conus to the bulbus 

 aortae, and so to the arteries. The valves in the veins are so 



