; THE FROG 12 
Sketch the floor and the roof of the mouth, twice natural size, 
labelling all the parts. 
IV. PRELIMINARY STUDY OF THE INTERNAL ORGANS. 
The work of this section should be done on a freshly-killed 
specimen. The method of cutting the frog open will be demon- 
strated at each table, but each student should perform the opera- 
tion on his own specimen. The inflation of the lungs and the 
bladder will be demonstrated. 
Place the frog on its back and with the smaller pair of scissors 
divide the skin in the ventral median line from the posterior to the 
anterior end of the body. 
How and where on the ventral surface is the skin attached? 
Observe the great lymph spaces beneath the skin, between the 
attachments. Turn the flaps of skin outward and pin them to 
the tray; to do this it will be necessary to sever certain of the 
attachments. 
Note (by feeling) the cross-shaped pectoral girdle, the center 
of which lies in the line uniting the bases of the two arms 
(examine the pectoral girdle in a preparation of the skeleton of 
the frog). The longitudinal bar of the cross is the sternum. 
Make a longitudinal incision through the ventral body-wall, a 
little to the left of the median line and extending from the 
posterior end of the body to the hinder end of the sternum; be 
careful not to cut the vein running along the median line. Be 
careful also not to injure the bladder (described below). Ob- 
serve the anterior abdominal vein in the middle line on the 
under surface of the larger flap. 
With your stronger pair of scissors carry the incision forward 
through the pectoral girdle to the level of a line joining the angles 
of the jaws. 
Under the sternum, which can be raised, is a membranous sac, 
the pericardium, containing the heart. Observe the position of 
the pericardium with respect to the large dark-colored lobed 
organ, the liver, filling the front part of the body cavity. Ob- 
serve the attachment of the pericardium to the ventral body-wall 
by a vertical membrane which also supports the liver. What 
relation does this membrane bear to the anterior abdominal vein? 
