A FROG 169 



The Internal Organs. Place the animal on its back in a dissect- 

 ing pan containing water, with its head away from you, and pin 

 it fast with a large pin through the tip of the jaw and one through 

 each of the four legs. Raise the skin of the belly with forceps, 

 and with scissors make an incision in it along the midventral line 

 the entire length of the body. 



Notice the looseness of the skin and the large space between 

 it and the underlying muscles. This space is a lymph cavity. 

 Note carefully the points where the skin is attached to the mus- 

 cles. Note the large blood vessels on the inner surface of the 

 skin ; these are the cutaneous veins and arteries. The blood is 

 brought to the skin to be aerated, an important part of the res- 

 piration of the animal being carried on through the skin. 



Through the semitransparent muscles in the region of the fore- 

 legs may be seen and felt a number of platelike bones and 

 cartilages. These form the pectoral (shoulder) girdle and the 

 breastbone, which support the fore limbs. In the midventral 

 line will be seen through the body wall a broad, dark line ; it is 

 the abdominal vein. 



Observe the ventral body muscles; see page 187. 



Lift up the ventral body wall with forceps, and with scissors 

 make a longitudinal incision through it in the median line the 

 length of the body, taking care not to cut the organs lying be- 

 neath. Pull the two flaps of the body wall gently apart and pin 

 them. Examine the organs which lie in the abdominal cavity, 

 but without disturbing any of them. 



If the animal is a male, or a female which is not breeding, the 

 most conspicuous organs will be the large, reddish liver and the 

 intestine. If it be a mature female, the dark-colored, granular 

 ovaries may occupy a large part of the space within the body 

 cavity ; in this case the ovaries should be removed so that the 

 other organs can be studied. 



Lying on the left side of the liver and wholly or partly con- 

 cealed by it is the elongated stomach. In front of the liver in the 

 median line is the conical heart within its membranous pericar- 

 dium. Lying between the lobes of the liver may be seen the small, 

 greenish, spherical gall bladder. 



