PHYSIOLOGICAL SERIES. 



tin 1 removal of the roof of the thalamoncephalon and 

 th<> upper part of the left hemisphere, and by the sagittal 

 division of the hinder parts of the brain and the turning 

 t,, one side of the left half (fig. 23). 



Fig. 23. 



LONG. ZONE 



SIR 



TUB.ACOUS. 

 LOB.LIN.LAT. 



MED.AUR 



OLF. BULB. 



Brain of Chinuera monstrosa (dissected). 



The brain is divisible into three well-defined regions : 

 (i.) A large anterior mass composed of two pairs of con- 

 tiguous lobes, the anterior of which are small and in 

 connection with the olfactory organs and the posterior 

 united together in the mid-line ; they are the olfactory 

 bulbs and cerebral hemispheres, (ii.) A remarkably long 

 and slender thalamencephalon. (iii.) The mid-brain, 

 errelx'llum. and medulla a considerable mass, remarkable 

 for its great dorso-ventral depth. This hinder part, owing 

 to the resemblance that the sides of the thalamencephalon 

 bear to olfactory peduncles, was formerly supposed to include 

 the cerebrum. A more detailed examination will show that 

 this brain, in spite of its obvious peculiarities, is, apart from 

 the cerebrum, -^ntially that of an Elasmohranch. The 

 olfactory bulbs are small and rounded in form; each is 

 feebly bilobed. They are separated from the hemispheres 

 l.y a furrow. Each hemisphere is fusiform, and is attached to 

 its fellow near the middle of its median surface by a delicate 

 lamina terminals continuous above and below with the roof 

 and the floor of the thalameneephulon. Slightly behind 

 the lamina a large semicircular opening (foramen of Monro) 

 leads on each side from the 3rd ventricle into a cavity that 



