120 



ANATOMY OF VERTEBRATES. 



Dissected brain of a Pigeon. 



XXXIV. 



produced, so as to cause a concavity transversely between them 

 and the Lateral borders, as shown in fig. 44. On the lower 

 part of the side of each hemisphere there is a depression which 

 corresponds to the ' fissura magna Sylvii,' and affords the sole 

 indication of a division into lobes. The hemispheres are con- 

 nected together by means of the round commissure, fig. 45, k. 

 The mesial surfaces of the hemispheres, which are in contact 

 mth each other, present stride which diverge 

 from the commissure. These surfaces are 

 composed of an extremely thin layer of medul- 

 lary substance, fig. 45, /, ^, forming the in- 

 ternal parietes of the ventricle, and extended 

 outwardly over the corpus striatum, ib. i. 

 Like its homologue in Rejjtilia and the mam- 

 malian embryo, it does not present the alter- 

 nate strioe of grey and white matter, which 

 suggested its name in Anthropotomy. This 

 cerebral ganglion is of great relative size in Birds, constituting of 

 itself almost the entire substance of the hemisphere, projecting 

 into the ventricle, ib. A, not only from below, but from the ante- 

 rior and outer sides of the cavity, 

 and being covered by a smooth 

 layer or fold of medullary matter, 

 f, which increases in thickness an- 

 teriorly. The ventricle does not 

 extend below the corpus striatum 

 to form an ' inferior horn,' or 

 ' cornu ammonis.^ A fold of pia 

 mater enters the bottom of the 

 cerebral ventricle and lies free in 

 the cavity : it is highly vascular, 

 and developes tufts containing plexi- 

 form loops of capillaries defended 

 by epithelium, the cells of which are 

 shoAvn at the marmn of the villi mao^nified in fior. 46. The vessel 

 forming the plexus choroides penetrates the ventricle beneath the 

 posterior part of the thin internal wall, and the lateral ventricles 

 communicate together there, and with the third ventricle. They 

 are continued anteriorly to the root of the olfactory nerve, which 

 is itself a continuation of the apex of the hemisphere. 



Just above the orifice of communication there is a smooth 

 flattened projection, rounded externally, which advances into the 

 ventricle from the internal Avail ; this represents a beginning of 



Tuft of the choroid plexus of the braii 

 Goose. XXXV. 



