I50 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



expanded, because the development of the cerebellum forces the 

 anterior parts of the two dorsal zones far apart, thus stretching the 

 plate into a thin and broad sheet. The floor plate, on the other hand, 



becomes greatly thickened and consti- 

 tutes the pyrairdds which pass in front 

 into the cerebral peduncles. Behind, 

 the medulla passes gradually into the 

 spinal cord, the line between medulla 

 and cord being marked, not by any 

 structural difference in the nervous 

 tissues, but by the posterior limits of 

 the skull. 



Ventricles. — Thus there are formed 

 from the three early divisions of the 

 brain, five regions — telencephalon, dien- 

 cephalon, mesencephalon, metenceph- 

 alon and myelencephalon — which oc- 

 cur in all vertebrates. These usually 

 retain in their interior the derivatives 

 of the cavities of the original three divi- 

 sions, which were but a continuation of 

 the central canal of the spinal cord. By 

 the outgrowth of the cerebral hemis- 

 pheres the cavity of the fore-brain 

 becomes divided into three chambers 

 Fig. 157.— Diagram of lower or ventricles, one lateral ventricle in 



half of brain, the cerebellum in , , . , , ^i xt_' j • 



- each hemisphere, and one, the tnird, m 



the 'twixt-brain, the lateral ventricles 

 being connected with the third by a 

 pair of narrower canals, the interven- 

 triciilar foramina (foramina of Monro). 

 In the higher vertebrates the primitive 

 ventricle of the mid-brain becomes re- 



position and represented as if 

 transparent, a, aqueduct; al, 

 acustico-lateralis lobe; c, corpus 

 striatum in floor of lateral ven- 

 tricle of telencephalon; cc, cen- 

 tral cana of cord; /, interven- 

 tricular foramen; i, infundibulum; 

 /, lamina terminalis; //, longitudinal 

 fissure; ms, mesencephalon; tnt, 

 metencephalon; my, myelenceph- 

 alon; r, rhinencephalon; p, 



pallium; v, visceral lobe; 3, third duced to a narrow tube, the aqueduct 



ventricle in diencephalon; 4, ., ■<.-.■> \ ^ u .. -^ 



fourth ventricle. or iter, but m the lower vertebrates it 



remains large and may extend up into 



the optic lobes where it is called the epicoele. The aqueduct is 



limited behind by the isthmus, but beyond this comes the very large 



fourth ventriclie below the cerebellum and extending back into the 



medulla, where it gradually narrows to the central canal of the 



