15G 
HELIX — CIRCULATORY SYSTEM. 
situate at the sides of the jieclal gland within the foot. The 
lateral pedal sinuses are only indirectly, but the visceral sinus is 
directly continnons with the great circular pulmonary sinus, which 
surrounds the base of the pulmonary chamber, and adjoins the rectum, 
Irom which it receives many small veins. 
From the circular i)uhuonary sinus the blood is distributed upon 
the sides and roof of the respiratory cavity, which is formed by a thin 
fold of the mantle and bears within or on its surface, especially 
anteriorly and along the right side, a very rich plexus of thin-walled 
blood vessels, within which the blood becomes o.xygenated by exposure 
to the air within the respiratory chamber. 
Fig. 310 . — Helix aspeesa from a dissection, after injection, to show the chief venous sinuses and 
the respiratory and renal capillary systems. The pulmonary* sac is cut close along the rectum and 
turned hack to show the whole pulmonary plexus at one view and the heart opened to show the 
interior of the auricle and ventricle. The venous vessels are all darkly shaded (after Howes). 
a.7'. aftereiu pulmonary' veins carrying the Mood for oxygenation to the pulmonary plexus from 
the circular pallial sinus, c./.a*., which receives its supply chiefly from the visceral sinus, v.s.^ and 
from the paired pedal sinuses, of which one is shown, ; ui. mantle margin; /.7 a pulmonary 
vein, whicli receives the arterialized blood from the re.spiratory plexus; r.p, renal capillary plexus. 
Tlie alfcrent or iucuiiiing and the efferent or dei)artiug veins alter- 
nate with great regularity and are in intimate connection by means of 
a very comi)lex and intricate series of delicate vessels, the efferent 
pulmonary vessels gradually uniting to form the great pulmonary vein, 
a large venous trunk on the roof of the respiratory cavity, conveying 
the arterialized blood direct to the auricle. A considerable volume 
of the blood, however, has a portal circulation within the tissues of 
the renal organ, during which the uric acid and other waste substances 
are eliminated from it, the blood thus purified entering the pulmonary 
vein by means of a large ami some small veins, without fully circulating 
within the iiulmonary plexus. 
