176 THE FROG chap. 



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 axiA sciatic swellings, ihe 

 filum terminate, 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 ^e. 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 beneath 

 them. 



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

 its dorsal side is the anterior choroid plexus, roofing in the third 

 ventricle ; and on its ventral side the infundibulum, to which the 

 Htuitary body is attached ; and, more anteriorly, the optic chiasma. 



5. The cerebral hemispheres, continuous in front, with 



6. The olfactory lobes, which are fused together 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 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 

 relations of the ventricles of the brain : — viz. , the fourth ventricle, the 

 iter and optic ventricle, and the third ventricle, which communicates 

 vith the lateral ventricle through the foramen of Monro. Sketch. 



