66 PHYSIOLOGICAL SERIES. 



conjunction with the similar vascular roof of the optic 

 lohes spreads out over a large part of the dorsal surface of 

 tlx> brain. These two choroid plexuses have been removed 

 together and mounted at the side with their under surfaces 

 >sed to show the complicated folding of their walls 

 (fig. 19, B). 



The roof of the medulla behind the open rhomboid fossa is 

 slightly thickened on either side of the mid-line, and here 

 gives origin to the hinder roots of the vagus (the roots 

 an- not shown in the specimen). Similar thickenings in the 

 lateral walls of the fossa give origin to the 7th and 8th 

 cranial nerves, and another pair of swellings in the floor of 

 the fourth ventricle, seen through the open rhomboid fossa, 

 are the motor nuclei of the 5th pair of nerves. The floor of 

 the fourth ventricle is indented in the mid-line by a sharp 

 furrow, on either side of which is a slight thickening due 

 to a pair of tracts (fasciculi longitudinales posteriores) that 

 connect the thalamencephalon with the cord and on their 

 \vay form connections with the motor nuclei of the cranial 

 nerves. They are particularly well marked in Fishes. 



The rhomboid fossa is bounded in front by a narrow but 

 slightly thickened lip the cerebellum. The mid-brain con- 

 sists above of a pair of very prominent rounded eminences 

 tho optic lobes. They contain a common ventricle 

 derived from the aqueduct of Sylvius and are superficially 

 separated from one another in the dorsal mid-line by a 

 groove. Their walls are composed for the most part of 

 nervous tissue from the outer surface of which the optic 

 ii'Tves are given off, but this is replaced at the anterior end 

 by an epithelial choroid plexus (the fore part of the speci- 

 men at the side) similar to that covering the rhomboid fossa. 

 Tin- removal of this plexus has exposed a minute opening 

 i ted at the anterior end of the optic lobes and bounded 

 in front by a band of commissural fibres (posterior com- 

 mi^ure) not externally visible that marks the boundary 

 line between the mid-brain and thalamencephalon or 

 primary fore-brain. 



Tin- thalamencephalon is remarkably deep from above 

 downwards. Its roof is epithelial in its anterior part and 



