216 ESSENTIALS OF ZOOLOGY 



thickened walls. All these four regions receive olfactory fibres 

 from the olfactory lobe, and all are continued into the thalamus. 

 The regions therefore externally may be regarded as being 

 the primordia of the pyriform lobe above and of the corpus 

 striatum below ; and those of the inner wall, of the hippocampus 

 above and of the paraterminal body below. The striatum 

 is continuous with the lower part of the thalamus or hypothala- 

 mus, and the hippocampus with the epithalamus. The other 

 two areas are continued likewise into the upper and lower 

 moieties of the middle part of the thalamus. 



As has already been made plain, the changes which take 

 place in the conversion of the forebrain into adult structures 

 are of a remarkable and important nature. Paired outgrowths 

 give rise to the hemispheres and to the retina of the eye. The 

 roof is protruded to form the pineal outgrowth and is continued 

 into the lamina terminalis, which is continuous again with the 

 floor. The floor presents the chiasma swelling, and is produced 

 into a pouch-like backwardly directed diverticulum, the 

 infundibulum, which becomes attached to the pituitary body. 

 The side walls are thickened to form the thalamus on each 

 side, and are put into communication by the transverse 

 anterior and posterior commissures. 



The optic lobes are prominent rounded outgrowths of the 

 midbrain or mesencephalon, and they contain a cavity com- 

 municating with the iter. The iter expands into the large 

 IVth ventricle, which in turn runs into the central canal of the 

 spinal cord. The triangular-shaped roof of the IVth ventricle 

 is bounded in front by the small, transverse cerebellum, and is 

 otherwise a thin covering indented by a plexus of blood-vessels. 

 The spinal cord extends from the foramen magnum to the 

 urostyle, where it is narrowed to form the filum terminale. 

 It is enlarged slightly in the regions of the brachial and lumbar 

 plexuses. It is flattened dorsoventrally, and possesses dorsal 

 and ventral fissures. 



Both the brain and spinal cord are invested by a thin 

 pigmented connective-tissue layer, the pia mater, which conveys 

 blood-vessels, and these expand into choroid plexuses by 

 indenting the thin roofs of the Illrd and IVth ventricles. 

 The dura mater is a similar membrane investing the skull. 



