EXPOSURE OF THE BRAIN OF MENOBRANCHUS. 417 



orbit until the left Lohus olfaciorius or the hemisphere is exposed. 

 Continue to remove that side of the cranium and the roof in very 

 small pieces and with great care. The widest part of the brain {me- 

 sencephalon, optici) lies opposite where the tympanum was removed, 

 and is liable to injury unless the adjoining cartilaginous capsule of 

 the internal ear is removed very cautiously. 



Caudad of the mesencephalon the brain is narrower and merges 

 into the myelon, which must be exposed by the removal of the 

 neural arches ; it will be necessary to cut away part of a thin car- 

 tilaginous plate upon the shoulder, the suprascapula (§ 383). 



Pass a scissors blade through the pharynx to the dextral angle 

 of the mouth and cut obliquely, so as to separate the cranium and 

 two or three vertebrse from the rest of the body. 



§ 1089. Exposure of the Brain of Menohranchus. — Instruments 

 and materials (§ 1088). With the arthrotome, cut the slvin upon a 

 transverse line just cephalad of the eyes. With the scissors, cut 

 caudad along a line just mesad of the eye on each side to a point 

 opposite the caudal gill. Raise and remove the flap so outlined, 

 noting that the skin adheres more closely than in the frog, and tliat 

 between it and the cranium there are considerable muscles. 



With the arthrotome scrape the muscles from the bone, begin- 

 ning at the cephalic end of the exposed area. Alternately ventri- 

 duct and dorsiduct the head so as to indicate the position of the 

 occipito-atlantal arthron. With the arthrotome, carefully pick up 

 the membrane between the atlantal neural arch and the cranium so 

 as to expose the metencephalon (medulla). 



With tlie nippers remove the neural arches of the first two or 

 three vertebrae, taking care not to wound the myelon. Then remove 

 the occiput in like manner, inserting the nipper blade but a very little 

 way. The larger part of the cranial roof is very thin and may often 

 be lifted in slivers upon the point of the arthrotome, but with large 

 individuals the nippers may be needed. Special pains should be 

 taken not to disturb the metatela, a pigmented and vascular curtain 

 just cephalad of the occiput, which sometimes adheres to the skuU 

 or is caught by the point of the instrument. 



When the dorsal aspect of the brain is exposed, with the scissors 

 cut away the left side of the head along the line of incision of the 

 skin, and then cut across the vertebral column and other parts 

 obliquely from the caudal end of the incision to the dextral angle 

 of the mouth. 

 37 



