PIXEAL GLAND. 



117!) 



corpora quadrigemuKt, and thus become a part of \\w. 

 enrolling stations for the nervous action which pro- 

 ceeds, according to Luys, between the surface-layers 

 of the cerebral hemispheres and the central gray matter 

 of tiie sjiinal column, i. e. Ix'twcen the psycho-intel- 

 lectual and the vegetative life. The 'twcenbrain, on 

 the otiier liand. has developed into the ojitic thalanii, 

 uliicli now, however, have a far greater importance 

 tiian tiiat of mere collectors of the visual sensations, 

 for hither, according to Luys (hg. 348), stream all 

 the sensations not yet idealised, but destined to the 

 consciousness. Here Luys distinguished between four 

 centres, wcll-deiined gangliar masses in each optic 

 thalamus (i), the foremost of which receives and trans- 

 mits the impulses of smell {20), -the second those of 

 sight [IS — 14), tlie third those of touch {8 — 9), the 

 fourth those of hearing [3 — 4). Before and outside 

 the optic thalarai lies the primitive floor of the cerebrum 



f stp Ihrh hi 



was a time \\\\vu it was su[iposc(l that tiie soul had a 

 fixed and s|)t<'ial station in some part of the brain, and 

 Descaktes assumed that the pineal gland was the seat 

 of the soul: — it had such a remarkably central situa- 

 tion, and it was solitary, unpaired. Had the soul dwelt 

 in a paired organ, said Descartes, we should always 

 have two ideas aljont the same thing, and there would 

 be a continual conflict to determine which side should 

 have the mastery. The researches of modern times 

 have certaiidy divested tiie pineal body of most of its 

 reputation from this point of view; but its historical 

 importance claims all the greater recognition. 



Before the transformation of the 'twcenbrain into 

 optic thalanii, even before the apjiearance of the rudi- 

 ment of the true optic thalanii, this part of the l)rain 

 grows laterally outwards in a vesicular form and sup- 

 plies the rudiments of the eyes and optic nerves, and 

 in a downward direction it forms an expansion known 



stp c r 



Fig. .340. Ripe female of the Lampern {Petromyzon fluviatiUs, forma branchinlis), uat. size. .1, from the left; B, forepart, from above; C, the' 



same, from below. 



1-. anterior connecting branch between the frontorostral canals; stp, posterior connecting branch; r, rostral branch; so, suborbital branch; 



m, mandibular branch; hn, mediolateral line; Id, dorsolateral line; Ibrs, superior branchiolateral line; Ibri, inferior br.inchiolateral line. 



(forebrain), the so-called corpora striata (2), to which 

 the manifestations of consciousness that have been de- 

 rived from the said sources and developed in the sur- 

 face-layers of the cerebral hemispheres are conducted 

 {14— 15— Id— 17: 9—10—11—12 and 4—o—G) for 

 tlieir tirst realisation, with that contribution to the 

 power and distinctness of the currents which is suiiplied 

 by the little brain, whose upper peduncles, according 

 to Luys, liy means of their jellow fibrils join tiie gang- 

 lion cells in the yellow nucleus of each corpus striatum. 

 Between the optic thalami and the corpora quadrigemina 

 lies a small swelling, the pineal gland {Ep in fig. 347), 

 the historv of whose transformations is among the most 

 remarkable. In man the nervous matter of the pineal 

 bod}^ probably plaj's the same part as that of the cor- 

 pora quadrigemina, but is of less importance. Yet there 



as the underbrain {Inf. in fig. 346); l)ut in the roof of 

 the 'twcenbrain there appear two thickenings of the 

 cerebral wall (behind Ep in fig. 347), situated side by 

 side and usually of uiic(iual size. These collections of 

 nervous matter become the rudiments of the true optic 

 thalami. Over the 'twcenbrain the cerebral wall has 

 grown out into a vesicular swelling {Ep in figs. 346, 

 347), which is at first flattened, but rises, becomes tu- 

 biform, and grows forward over the brain. This is the 

 first appearance of the pineal gland in the lower verteb- 

 rates, and in them it attains a considerable. develop- 

 ment. In the Lampreys it is even externally visible 

 through the skin as a light, round spot behind the 

 tubular aperture of the nostril (fig. 349, B). 



The pineal gland belongs, as mentioned above, to 

 that division of the brain with which the orifrin of the 



