172 COMPARATIVE ANATOMY 



the wall consists of several layers and forms a pigmented retina, 

 with which the more or less rudimentary nerve is continuous. The 

 vesicle is surrounded externally by a connective-tissue capsule, and 

 in many cases the integument and connective-tissue immediately 

 overlying the vesicle is pigmentless and transparent, forming a 

 kind of cornea. Traces of a vitreous body have also been observed. 

 Various degrees of reduction of the different parts as they occur 

 e.g., in Hatteria (Fig. 141), are seen amongst Lizards. (See also p. 

 155). Traces of a parietaleye, with lens and pigment, have also 

 been observed in the embryo of the Viper (Pelias berus). 



In the mid-brain tho two well-marked optic lobes may 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. 140), in which it 

 consists of a thicker median, and two lateral portions, while in 

 other Eeptiles, and more particularly in Lizards, it is not much 

 more highly developed than in Amphibians. The medulla ob- 

 longata has a marked ventral flexure. 



Birds. — The basal ganglia (corpora striata) of the hemispheres 

 reach a relatively larger size in Birds than in any other Vertebrates, 

 while the differentiation of the cortex and commissures does not 

 show any marked advance on that seen in Reptiles. 



The different parts of the brain overlie one another much 

 more markedly than in any Reptile, and the hemispheres are 

 much larger relatively, covering over the thalamencephalon and part 

 of the mid-brain (Fig. 142). The olfactory lobes are short and 

 conical. The distal, enlarged end of the pineal body extends as 

 far as the dura mater, and the structure of the internal part of the 

 organ resembles that of a tubular gland, penetrated by fibrous tissue 

 and blood-vessels. There is no trace of a parietal organ. 



The cerebellum consists of a well- developed and folded median 

 lobe, and of two lateral portions (flocculi), which vary much both in 

 form and size. Posteriorly it completelj'' covers the fourth ventricle. 

 The two optic lobes are separated from one another and pressed 

 downwards, so as to lie on the sides of the brain in the angle between 

 the hemispheres, cerebellum, and medulla oblongata, and they are 

 connected by a broad commissure. The venti-al side of the hind- 

 brain shows a marked flexure, bending upwards to the spinal cord. 

 Mammals. — The brain in embryo Mammalia is very similar 

 to that of the Sauropsida, but its later differentiation — more 

 particularly that of the pallium — gives it a very special character. 

 The cortex becomes much more highly differentiated, and in 

 many Mammals is more or less highly convoluted (Figs. 144, 146), 

 giving rise to gyri and sulci (p. 154). In others, again, the surface 

 of the hemispheres remains smooth (Fig. 143), but a subdivision 

 into lobes (frontal, parietal, temporal, &c.) can always be recog- 

 nised to a greater or less extent, and the hemispheres are 

 relatively so large as to cover over the more posterior parts of the 

 brain ; in some of the lower forms, the mid-brain can still be seen 



