14 



BRAIN 



the least modified part ; it resembles the spinal cord most closely 

 in histological structure, and from it emerge all the cranial nerves to 

 which a segmental value is usually attached, except the oculomotor 

 and pathetic. It always retains a considerable cavity, metacoele or 

 4th ventricle. Its roof is never entirely nervous, and forms a 

 vascular membrane, the choroid plexus. The roof of the meten- 

 cephalon becomes thickened as the cerebellum, which acquires a 



Fin. 7. 



Ventral view (A) and dorsal view (B) of the brain of Raja hatis. Dorsal view (C) of the 

 brain of Acanthias vulgaris, showing tin' internal cavities shaded. «, auditory nerve; 6m, 

 buccal nerve ; ca, <-a\ ity of mid-brain ; cct, lateral cavity of corpus restiforme ; cfb, cavity of 

 fore-brain, "cerebral hemisphere" ; e.o.l, cavity of olfactory lobe ; cop-.l, cavity of optic lobe ; 

 er, cerebellum; elf, corpora restiformia; d.r, dorsal root of spinal nerve ;/. 6, fore-brain (pros- 

 enci']ihalcin) : J'.r, fourth vi-ntricle ; ;//, glossopharyngeal nerve; Inn, hyoinandibular nerve ; if, 

 infundibulum ; l.c, labial cavity; l.i, lobus inferior; l.t, lamina terminalis ; m, medulla 

 oblongata ; o.l, olfactory lobe ; os, olfactory tract ; op.l, optic lobe ; pr, profundus nerve ; psp, 

 prespiracular branches of facial : ;.op, superioi ophthalmic branches of facial ami trigeminal ; 

 sp.c, spinal cord ; s.v, saccus vasculosus ; th, diencephaloti (thalamencephalon) ; t.v, third ven- 

 tricle ; y, vagus nerve ; v.r, ventral root of spinal nerve. 



large size and complicated structure in the higher Pisces and higher 

 Tetrapoda. The mid-brain remains undivided. The cavity it 

 encloses is known as the aqueductus of Sylvius, or mesocoele. 

 Above and at the sides the wall thickens into the prominent optic 

 lobes, from which pass fibres to the optic tract. Below, a bundle 

 of fibres develops into the crura cerebri. From this mesencephalon 

 issue the oculomotor and pathetic nerves. 



