468 ZOOLOGY. 



before trying to remove any part of the bone. Then, using 

 a bone-forceps if you have one, or, if not, a strong knife, 

 start cutting through the bone at the conspicuous raised 

 pentagonal area where the inter-parietal and supra-occipital 

 meet : the bone is spongy here, and the cerebellum lies 

 underneath. Starting from here, go on to expose the 

 cerebral hemispheres in front and the bulb and spinal cord 

 behind. In the rest of the- dissection do not try to remove 

 the brain from the head, but cut away the rest of the head 

 from the brain. Cut away nose and jaws, remove the eyes. 

 and gradually break away the bone all round the brain and 

 beginning of the spinal cord, leaving bits of bone attached 

 at any point where you do not feel safe in removing them. 

 Then place in pure formalin, in a wide-mouthed bottle, with 

 a lump of cotton-wool at the bottom for the brain to rest on. 

 After a few hours the brain will be harder : take it out 

 and wash it (have plenty of water at hand to keep the 

 formalin off your fingers), and remove as much more of the 

 bone as you can. Also remove very carefully the tough 

 membrane that surrounds it don't tear it, but cut it away 

 in pieces with scissors. Put the brain back in formalin 

 for a few days. When well hardened keep in methylated 

 spirit. It will be in a very good state either for dissection 

 or for cutting sections. 



The frog and dogfish are so well described in Marshall's 

 books that we need say little here about their dissection. 

 The student should not fail to cut a series of transverse 

 sections of the dogfish. Before such can be made of the 

 frog it must be soaked in chromic acid to soften the bones. 

 The frog's skeleton may be prepared in the same way as the 

 rabbit's, but great care must be taken not to lose the small 

 bones. The dogfish's may be prepared similarly, but 

 must not be boiled too much, as it consists of cartilage, 

 not bone. 



The dissection which usually gives most trouble in the 

 dogfish is that of the afferent branchial arteries. A very 

 neat dissection of them can be made by the following 

 method. Remove the skin from the ventral region be- 

 tween the pectoral girdle and the mouth. Slice away the 

 median ventral part of the pectoral girdle and adjoining 



