ae, THE VASCULAR SYSTEM OF THE FROG. 
Chap. IIL.—THE VASCULAR SYSTEM OF THE FROG. 
The vascular system is a closed system of tubes or vessels 
filed with blood, and ramifying throughout all parts of the 
body : its main parts are (1) the heart, which by its contractions 
is continually driving the blood round and round the system of 
vessels : (2) the arteries, which are the vessels taking the blood 
from the heart to all the different parts of the body: (3) the 
veins, which return the blood from those parts back to the 
heart: and (4) the capillaries, a system of very small vessels 
connecting the arteries and veins together. 
A. The Heart. 
Pin down the frog on tts back and open the body cavity as before, 
taking especial care to preserve the anterior abdominal vein. 
Dissect the vein carefully from the body-wall, and pin out the flaps. 
Open the pericardial cavity: examine and draw the heart in 
situ, showing 
1, The divisions of the heart. 
i, The auricles: anterior, thin-walled and dark coloured 
owing to the blood being seen through their walls. 
On close examination the division into right and 
left auricles can be seen. 
ii. The ventricle: posterior: paler in colour owing to 
the greater thickness of its walls: conical in 
shape, with the apex pointing backwards. 
iii, The truncus arteriosus: a cylindrical body arising 
from the right anterior border of the ventricle, 
and running obliquely forwards across the 
| auricles. 
Lift up the ventricle and turn ut forwards so as to expose 
iv. The sinus venosus: a thin-walled sac lying on the 
dorsal side of the auricles and ventricle, and 
receiving the three large vene cave, 
2. The pulsation of the heart, 
a. Note that the contractions of the heart continue some 
time after the frog has been killed, or even after the 
heart is completely removed from the body. 
