IV.] THE COMMON SNAIL. 293 



introduce a small syringe or medicine-dropper filled with 

 injecting material. Seize the vessel and the inserted nozzle 

 between the finger and thumb of the left hand, and inject. 

 If this simple operation be carefully performed, all the 

 leading vessels which carry aerated blood will be injected. 



1. Lay open the pulmonary-sac, and note : 



a. The efferent pulmonary vessels ; a uniformly arranged 

 series of trunks bringing in the blood from the 

 lung-sac ; they unite to form a large pulmonary 

 vein which enters the auricle. 



b. The efferent renal vessels ; two or three small trunks 

 conveying the blood from the excretory organ to 

 the pulmonary vein. 



During the process of injecting these become very 

 rapidly filled, and in the exercise of the pressure 

 necessary to inject the whole arterial system their 

 walls are not unfrequently ruptured. 



2. Open the heart (cf. Sect. C. i e). Note the thick 

 muscular wall of the ventricle, and the auriculo-ven- 

 tricular valves so disposed as only to admit of a 

 passage of the blood from the auricle to the ventricle. 



3. The arterial system. Remove the right half of the 

 body-wall and visceral sac, and together with them 

 the rectum and greater portion of the genital ap- 

 paratus cutting the sacculated hermaphrodite-duct 

 across at about its middle. 



There will thus be exposed the great arteries. Work 

 them over in the following order : 



a. The aortic trunk, a very short vessel, arising from 

 the base of the ventricle. It lies to the left of the 

 head of the first coil of the intestine, immediately 



