176 THE FROG CHAP. 



Fig. 52) which pass backwards for some distance before 

 making their exit from the neural canal ; and also the 

 ganglia on the dorsal roots, lying just outside the canal and 

 each hidden in a whitish calcareous body in this region 

 (Fig. 51, C). (The ganglia, however, can be more easily 

 made out at a later stage.) Then sever the nerves very 

 carefully from the brain and spinal cord and remove the 

 whole central nervous system from the neural canal : it is 

 best examined after hardening in formaline or spirit. Lay 

 it in a dissecting-dish, under water, and make out its several 

 parts as follows : 



a. The spinal cord. 



1. Note its cylindrical form, the brachial and sciatic 

 swellings, the filum terminale, and the dorsal and ventral 

 fissures. 



2. Examine a transverse section of the spinal cord, 

 prepared as described on p. 136, under the low power of the 

 microscope, and make out the dorsal and ventral fissures, the 

 central canal, and the relations of the grey and white matter 

 (Figs. 48 and 52). Sketch. 



b. The brain (Fig. 49). 



Beginning from the posterior end of the brain, where it 

 passes into the spinal cord, make out its several divisions as 

 follows : 



1 . The bulb or medulla oblongata, with the posterior choroid 

 plexus on its dorsal side : remove the latter, and notice that 

 it roofs over the cavity of the fourth ventricle. 



2. The small ledge-like cerebellum. 



3. The two rounded optic lobes, and the crura cerebri j 

 beneath them. 



4. The diencephalon, formed of a right and a left optic 

 thalamus. On its dorsal side is the anterior choroid plexus, 

 roofing in the third ventricle ; and on its ventral side the 

 infundibulum, from which the pituitary body easily becomes 

 detached ; and, more anteriorly, the optic chiasma. 



5. The cerebral hemispheres, continuous in front, with 



6. The olfactory lobes, which are fused in the middle line. 

 Sketch the whole central nervous system from above, and 



also the brain from below and from the side. 



With the small scissors or a sharp scalpel, snip off a small 

 piece of the wall of the hemisphere and optic lobe of one 

 side say the left, so as to expose the lateral ventricle and 

 the optic ventricle (Fig. 50). Then with a sharp scalpel 

 divide the whole brain into two by a longitudinal vertical 



