PRACTICAL DIRECTIONS 177 



cut very slightly to the left of the middle line, so as to reduce 

 it to a longitudinal section (Fig. 49, D). Examine the cut 

 surface of the right side under water, and make out as much 

 as possible of the commissures and of the relations of the 

 ventricles : viz., the fourth ventricle, the iter and optic 

 ventricle, and the third ventricle, which communicates with 

 the lateral ventricle through the foramen of Monro. Sketch. 



II. The Peripheral Nervous System. 



a. The spinal nerves. 



Fasten out a frog with the ventral side uppermost, and 

 remove the heart, enteric canal, reproductive organs, 

 kidneys, and lungs with great care, leaving behind most of 

 the systemic trunk and the dorsal aorta (Fig. 51). (One 

 of the specimens already dissected will probably serve the 

 purpose if the previous directions have been accurately 

 followed.) Note the spinal nerves passing outwards from 

 the vertebral column on either side, and the calcareous 

 bodies close to their points of exit, covering up the ganglia 

 of the dorsal roots (p. 163). If the centra of the vertebrae 

 are removed, the nerve-roots and their origins from the 

 spinal cord can be made out : the removal of the centra is 

 rendered easier if the frog is first decalcified by being placed 

 in 10 per cent, nitric acid for twenty-four hours and then 

 thoroughly washed in running water. 



Confine your attention to the large ventral branches of the 

 ten pairs of spinal nerves, as described on pp. 160 162. 



b. The sympathetic nerves (Figs. 51 and 53). 



Examine the systemic trunk and dorsal aorta carefully 

 with a lens. Closely connected with it will be seen on either 

 side a sympathetic nerve-cord, covered by pigmented con- 

 nective-tissue. Carefully dissect the cord away from the 

 aorta, and note the ganglia and the branches (rami com- 

 municantes) connecting them with the spinal nerves. Sketch 

 the spinal nerves and sympathetic. 



c. The cerebral nerves (Fig. 53). 



The dissection of these in a frog is not an easy task for a 

 beginner, and it is best to examine those of a larger animal 

 (e.g., dogfish) before attempting it (see Part II, p. 479)- 

 The origin of some of the nerves from the brain and the 

 apertures through which certain of them pass out from the 

 skull, have already been seen. 



III. The Microscopic Structure of Nervous Tissue. 



a. Examine your transverse section of the spinal cord 



PRACT. ZOOL. N 



