28 ' GENERAL BIOLOGY 



m. Internal Anatomy. 



Fasten the frog in the dissecting pan on its back, and slit 

 the skin the entire length of the body in the median line. 

 Is the skin loosely or closely attached to the body? The 

 spaces that are found beneath the skin are lymph spaces. 

 Laying back the skin observe the walls of the abdomen, 

 made up of muscles. Determine the points of attachment 

 of these muscles. In what directions do the fibers of the 

 muscles extend on the ventral wall, and on the side wall? 

 The ventral muscles are the straight abdominal, and those 

 of the side walls are the oblique abdominal muscles. Also 

 observe the group of pectoral or breast muscles in the region 

 of the arm. Is this one muscle or a group of muscles ? What 

 is the probable function of these muscles? 



Draw the ventral surface of the frog, showing the muscles 

 just studied. If time permits, the muscles of one of the 

 hind legs may be studied and a drawing made. 



Cut through the body wall, taking care not to injure the 

 organs lying beneath. Notice that the internal organs lie 

 in a large cavity, the body cavity or ccelom. Pin back the 

 flaps and observe the following organs which are exposed: 



1. Heart. In the median line beneath the pectoral girdle. 

 It has two thin-walled auricles and a thicker-walled ven- 

 tricle, the whole being enclosed in a delicate sac, the peri- 

 cardium. If the heart is beating record the order of the 

 pulsations of the different chambers. From the ventricle 

 a large vessel, the truncus arteriosus, extends obliquely for- 

 ward over the auricles. On the dorsal side of the heart is 

 a thin, triangular sac, the sinus venosus, into which the blood 

 comes before entering the heart. Does the sinus com- 

 municate with the auricles or the ventricle? 



Make drawings of the heart to show these points. 



