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 the systemic trunk and 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 

 nefves 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 p. 446. 



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 communicantes) connecting them with the spinal nerves. Sketch 

 the spinal nerves and sympathetic. 



c. The cerebral net ves (Yi^. 53.) 



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

 and directions for their examination in larger animals will be given in 

 subsequent chapters. The origin of some of them 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 (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 



