STRUCTURE OF THE ENCEPHALON. 



201 



matter of the vesicular wall. There is also another vascular 

 development inwards of the pia mater, between the cere- 

 bellum and the medulla oblongata, the choroid plexus of the 

 fourth ventricle. In this brain we miss the hypoaria of the 

 brain of fishes; but it may be noted that, close to the position 

 where they might be expected, there is seen in the interior a 

 thickening at the sides of the mesial canal, where -it dips 

 down at its termination. The optic nerves at their decussa- 

 tion are partially blended in an optic commissure, and behind 

 that point are inseparable from the brain, and named optic 

 tracts. 



In the brain of a bird, for example the turkey, the cere- 

 bellum is no longer a mere hollow 

 vesicle; its cavity is minute, its 

 surface covered with grey matter, 

 and thrown into deep transverse 

 laminae. The optic lobes project 

 laterally, and even downwards, in- 

 stead of upwards. There is a 

 thorough decussation of the optic 

 tracts. The hemisphere - vesicles 

 when opened are seen to be covered 

 with a very thin lamina, and filled . 

 up by the projection upwards of the Fig. 105. BRAIN OF A 

 corpora striata, so that what are usu- TURKEY. a Medulla 

 ,, , ,, ', . -, r i i oblongata ; &, cerebel- 



ally called the hemispheres of birds, lum . c> op tic lobe; d, 

 may be said to consist principally of 

 corpora striata. Between the hemi- 

 sphere-vesicles and the optic lobes 

 there is a thick neck of substance, but 110 very important 

 mass deserving a name. 



149. If we pass now to a mammalian brain in a foetal stage, 

 such as the lamb's brain in fig. 106, we have little difficulty 

 in recognising corresponding or homologous structures. The 

 medulla oblongata and cerebellum are obvious. In front of 

 the cerebellum are the optic lobes, placed superiorly as in 

 reptiles and fishes, and constituting the structures divided in 

 mammals by a crucial depression into four parts, and called 

 the corpora quadrigemina. In front of these are the hemi- 

 sphere-vesicles containing the corpora striata; but a little 



roof of hemisphere re- 

 flected to show corpus 

 striatum; e, optic nerve. 



