x PRACTICAL DIRECTIONS 177 



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 calcareoiis 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 connective-tissue. Carefully dissect the 

 cord away from the aorta, and note the ganglia and the branches 

 (rami conununicantes] connecting them with the spinal nerves. Sketch 

 the spinal nerves and sympathetic. 



c. The cerebral nerves (Fig. 53). 



The dissection of these in the 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). 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. 



in. The Microscopic Structure of Nervous Tissue. 



a. Examine your transverse section of the spinal cord (Fig. 48) under 

 the high power of the microscope, and observe 



1. The nerve-cells, present in the grey matter only (compare Fig. 54, 

 A). Note their branched form and their nuclei ; the larger motor cells 

 are seen in the ventral horns of the grey matter. Sketch. 



2. The nerve-fibres, in both grey and white matter, cut across trans- 

 versely as well as in other directions, and each showing a deeply- 

 stained central neuraxis. Sketch. 



PRACT. ZOOL. N 



