DISSECTION OF A FROG. 43 



division. Then follow each trunk. The right one soon 

 divides into three branches; the anterior is the carotid, the 

 middle the aortic arch, the third the pulmonary artery. 

 How does the trunk of the left side differ ? 



Trace the carotid arch ; where does it go ? What be- 

 comes of the aortic arch ? Do you find a dorsal aorta ? 

 On which side of the alimentary tract should the dorsal 

 aorta be (p. 13) ? To what organs is the pulmonary artery 

 distributed ? Do you find anything to compare with the 

 ventral aorta (p. 13) and afferent and efferent branchial 

 arteries ? Draw the circulatory system as made out. 



Place a drop of blood of the frog on a slide, cover it 

 with a cover-glass, pressing it well down, and examine 

 under the higher power of a microscope. What is the shape 

 of the corpuscles ? Are all alike in shape and size ? Stain 

 with fuchsin (see Appendix) and study again. Are all 

 parts equally stained ? 



Split the skin along the back and pull it away. Find the 

 point where the head joins the back-bone; and beginning 

 here, with a strong pair of scissors cut away the roof of the 

 skull bit by bit, taking great care not to injure the brain. 

 Then in the same way cut away the neural arches of the 

 vertebra. This will expose the brain and spinal cord. The 

 later work will be more easily followed if the animal be put 

 for a day or more in 70$ alcohol. 



In the spinal cord notice the spinal nerves given off at 

 regular intervals on either side. How many are there? 

 What relationship do they bear to the bodies of the verte- 

 brae ? Examine these spinal nerves more closely, and see if 

 each is double (has dorsal and ventral roots). Follow one 

 out by carefully cutting away the bone, and see where the 

 roots unite. Has either root an enlargement (ganglion) ? 

 Look in the dorsal part of the body- cavity for these spinal 



