68 lorRNAi. OF Co^tPARATIVF. Neurology. 



corpuscles. These nerve cells are faintly stained and have a 

 large, clear, granular, sub-spherical nucleus and a dense 

 nucleolus. They are about seven micro-millimetres wide and 

 from 13 to 15 micro-millimetres long. 



3. Extending along the ventricle, a narrow gelatinous 

 layer of Deiter's corpuscles. 



Paricto-ocripital Lobe (Plate \'I, Figs, i, 5: P. O. L). — 

 This lobe constitutes the caudad portion of the lateral division 

 of the mantle. It forms the greater part of the caudad por- 

 tion ot each hemisphere and extends from the occipital lobe 

 almost to the caudo-lateral border of the prosencephalon. 

 Entad this lobe is bounded by the basi-occipital lobe and the 

 corpus striatum, laterad it is bounded by the parieto-frontal 

 lobe, ectad it is superficial. In some types the lateral ven- 

 tricle penetrates this lobe. 



In the avian prosencephalon the parieto-occipital lobe is 

 much nearer the meson than is the corresponding lobe of the 

 reptilian brain.(') It is now almost universally admitted 

 that birds and reptiles have been evolved from the same 

 primitive group of vertebrates. If this be true, a mesal 

 revolution of the caudad portion of each hemisphere is the 

 only phenomenon that consistently accounts for the mesal 

 position of the pai^ieto-occipital lobe in the avian brain. 



Structure. — This lobe consists of the following parts: 



1. A narrow outer cell-less layer of free cortex. 



2. A wide, irregular, inner layer of nerve cells. This is 

 \h.c parieto-occipital nidiilus. It consists of a large number 

 of Deiter's corpuscles and large pyramidal cells. These cells 

 vary in length from 22 to 26 micro-millimetres. Their sides 

 are either convex or straight. The apex of each cell is 

 extended into a long process, while the base is supplied with 

 several shorter processes. The apex process is often several 

 times as long as the cell. These cells stain densely and have 

 an elongated dense nucleus and denser nucleolus. Occasion - 



1 See I'rol. Heiiiuks Paper uii '■ Topography and Histology of itic Brains ef Cer- 

 tain Reptiles." Supra p. 14. 



