388 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the pineal body (pin) projecting forwards from it. On each side of the 

 pineal body, and therefore slightly outside the plane of the section, the 

 brain-roof thickens to form the habenular ganglion Qi.g), the two ganglia 

 being connected by the habenular (or " superior ") commissure Qi.c) 

 which lies in front of the root of the pineal body. Apart from the 

 habenular ganglia the roof of the thalamencephalon is in great part thin 

 and membranous. The floor of the thalamencephalon is seen to dip down- 

 wards and backwards to form the infundibulum, the solid mass fused 

 with its tip being the pituitary body. The cavity of the infundibulum 

 is demarcated in front by a prominent transverse ridge (cK) — the optic 

 chiasma. The deep recess in front of the chiasma is the optic recess. 

 The front wall of the thalamencephalon is traversed by a large and 

 important mass of nerve fibres, forming a bridge between the two hemi- 

 spheres and known as the anterior commissure {a.c), and just above this 

 the brain-wall projects outwards in the median plane as the paraphysis 

 (par) — an organ of unknown significance. 



The hemispheres, like the habenular ganglia, are paired organs and 

 would not therefore appear in a strictly sagittal section, but to make 

 Fig. i68 complete the hemisphere (c.H) is shown as it would appear in 

 a section somewhat to one side of the mesial plane. It will be seen that 

 the hemisphere contains a wide cavity — the lateral ventricle — and that 

 the wall enclosing this is of fairly uniform thickness. In particular it 

 will be noticed that the roof (pallium, or mantle of the hemisphere) is 

 thick and well-developed. 



It has already been mentioned that certain parts of the brain-wall — 

 namely the roof of the Third and Fourth ventricles — are reduced to the 

 form of a thin membrane. The object of this would appear to be to 

 facilitate diffusion-processes between the lymph filling the cavities of the 

 central nervous system (cerebro-spinal fluid) and the blood, as a rich 

 network of blood-vessels (choroid plexus) lies in close apposition to these 

 thin portions of brain-wall. Now in the case of the hemisphere a small 

 portion of its wall, close to its hinder end, similarly assumes this form and 

 together with its plexus of blood-vessels bulges inwards into the hemi- 

 sphere and eventually forms a complex and irregular lateral plexus (}.p), 

 with a very large area of surface through which diffusion can take 

 place. Clearly we have here an arrangement for ministering to the 

 needs — respiratory, nutritive and excretory — of the hemisphere wall. 



As shown in the figure an extension of the lateral plexus projects 

 back into the third ventricle. In the higher vertebrates this is con- 

 tinuous across the mesial plane and is termed the velum transversum. 

 The Lung-fishes possess the same equipment of sense organs as the 



