THE EKCEPHALON. 59 



the organs are supplied by spinal nerves ; and in Malapie- 

 rm-us, tlie nei^ve consists of a single gigantic primitive fibre, 

 which subdivides in the electrical organ. 



The ordinary Rays possess organs of much the same 

 stmcture as the electrical apparatus, at the sides of the tail. 



Tlie Nervous System: the Encephalon. — In all vertebrated 

 animals except Amphioxus, the brain exhibits that separa- 

 tion into a, fore-brain, mid-brain, and hind-brain, which results 

 from its embryonic division by two constrictions, into the 

 three thin-walled vesicles — the anterior, middle, and pos- 

 terior cerebral vesicles — already mentioned. The cavities 

 of these vesicles — -the primitive ventricles of the brain — 

 fi'eely communicate at first, but become gradually dimi- 

 nished by the thickening of their sides and floors. The cavity 

 of the anterior vesicle is, in the adult human brain, repre- 

 sented by the so-caMed third ventricle ; that of the middle 

 vesicle, by the iter a tertio ad quartum ventriculum; that of 

 the posterior vesicle, by the fourth ventricle. 



The floor and sides of the posterior vesicle, in fact, thicken 

 and become the medulla oblongata ; together with the j^ons 

 varolii, in those animals which possess the latter structure. 

 The posterior part of the roof is not converted into nervous 

 matter, but remains thin and attenuated ; the epend/yma, or 

 lining of the cerebral cavity, and the arachnoid, or serous 

 membi'ane which covers the brain extei-nally, coming nearly 

 into contact, and forming, to all appearance, a single thin 

 membrane, which tears with gi-eat readiness, and lays open 

 the cavity of the fourth ventricle. Antei'iorly, on the other 

 hand, the roof becomes converted into nei-vous matter, and 

 may enlarge into a complex mass, which overhangs the 

 posterior division, and is called the cerebellum. The pons 

 varolii, when it exists, is the expression of commissural 

 fibres, which are developed in the sides and fioor of the 

 anterior part of the posterior cerebral vesicle, and connect 

 one haK of the cei'ebeUiim with the other. 



Thus the hind-brain differs from the posterior cerebral 

 vesicle in being differentiated into the medulla oblongata 



