BRAIN 221 



with their choroid plexuses are much more distinct in many cases 

 (e.g. Hatteria, Chelonia, Crocodilia). 



The olfactory lobes may be closely applied to the hemispheres 

 (e.g. Anguis, Amphisbaena, Typhlops), or may consist of a well- 

 marked olfactory tract, passing anteriorly into an olfactory bulb 

 from which the nerves of smell arise (e.g. Hatteria, Lacerta, 

 Crocodilus). Olfactory ventricles are usually present. 



The diencephalon is always depressed, and is hardly, or not at 

 all, visible from the dorsal side. A distinct hypophysis and in- 

 fundibulum as well as an epiphysis are present, and in most 

 Lizards the parietal organ (cf. p. 202) retains more or less distinctly, 

 even in the adult, the structure of a median eye. 1 



This parietal eye (Fig. 168) is situated in the parietal foramen 

 of the skull, and is in close connection with the more posteriorly 

 situated pineal organ, though in the embryo the nerve which 

 supplies it is seen to arise independently from the brain, in front 

 of the pineal outgrowth. It has the form of a vesicle, the dorsal 

 wall of which may become thickened to form a transparent lens- 

 like body, while the rest of the wall consists of several layers and 

 forms a pigmented "retina," with which the more or less rudi- 

 mentary nerve is continuous. The vesicle is surrounded by a 

 vascular connective tissue capsule, and in many cases the integu- 

 ment immediately overlying it is pigmentless and transparent, 

 forming a kind of cornea. Traces of a vitreous body have also 

 been observed. 2 Various degrees of reduction of the " retina " 

 and other parts as they occur, c.t/. in Hatteria, are seen amongst 

 Lizards (e.g. Lacerta, Anguis), and the organ may be recognised in 

 a simpler form in embi'yo Snakes. 



As in all the Amniota, two chief divisions can usually be recog- 

 nised in the infundibulum of Reptiles : a dorsal vascular and 

 glandular body, corresponding to the saccus vasculosus of the 

 Anamnia, and a more ventral infundibular portion, the glandular 

 character of which is still retained to some extent in the Sauropsida, 

 but there is no opening into the ventricular cavity. 



In the mid-brain the two well-marked optic lobes in some cases 

 show indications of a further subdivision into four ; from them the 

 optic tracts pass downwards and forwards to the chiasma. The 

 cerebellum is relatively small, except in the Crocodilia (Fig. 

 167), in which it consists of a thicker median lobe, and of two 

 lateral portions. The medulla oblongata has a marked ventral 

 flexure. 



Birds. The avian brain (Fig. 169) is of a very peculiar type : 

 it has few points of resemblance to that of Mammals, and is very 

 different from that of Reptiles, though especially as regards its 



1 A parietal organ is wanting in Gecko, Ameida and Tejus, and there is no 

 pineal organ in the Crocodile. 



! The parapliysis gradually extends beneath the epiphysial outgrowth, and 

 forms a sort of cushion under the parietal eye. 



