490 ANATOMICAL TECHNOLOGY. 



Syri. — Thalamus opticus, thalamus nervi optici, ganglion cerebri posticutn, eminentia 

 magna cerebri. 



The tlialami form the walls (lateral parietes) of the diacoelia, and are connected by the 

 medicommissura. 



Most of the important features of the thalami are described in the sections referred to ; 

 there is one, however, which may properly be insisted upon here, inasmuch as it is not 

 commonly recognized ; viz., its complete exclusion from the procaelia in the cat. 



In most works upon Descriptive Anatomy, the thalami are mentioned as appearing in 

 the " lateral ventricles," and in Gray (A, Fig. 364) and Quain (A, II, Fig. 383), the surfaces 

 of the striata and thalami are apparently similar and continuous. Now the fornix (including 

 the hypocampae and fimbriae) of man is relatively narrower than that of the cat, and it is 

 quite possible that in the adult, even without the rupture of the membranes at the rima, 

 a part of the dorsal aspect of the thalamus may appear in the proccelia ; if so, however, 

 that part of the surface must be covered by the proocelian eadyma, and the line of reflec- 

 tion (ripa) upon the proplexus should be represented. Upon these points our materials 

 for observation do not enable us to speak more fully. 



But in the cat and dog, we are prepared to state emphatically (§ 1083), that the mar- 

 gins of the rima are in close apposition, excepting for the intruded elements of the proplexus, 

 and that in these animals and in other mammals examined by us, no portion of the thala- 

 mus appears in the procoelia or enters into the formation of its floor ; any statements to 

 the contrary should not be accepted without detailed descriptions and figures. 



§ 1326. Tractus opticus, Tr. op.— Fig. 116, 118; PI. II, Fig. 3; PI. Ill, Fig. 9, 11 ; 

 § 1144. Gray, A, 638 ; Quain, A, II, 533. 



The subeylindrical fibrous band whi )h arises from the diencephalon, and perhaps from 

 the mesencephalon, and meets its platetrope cephalad of the Tuber cinereum to form the 

 chiasma. The larger part of the tract seems to be a direct continuation of the praegenicu- 

 latum, but we have not traced the fibers in detail. See Chap. XI. 



§ 1327. Tractus postrhinalis, Tr. prh.—Fig. 116 ; PI. I, Fig. 2 ; PI. II, Fig. 3. 



This and the following name were proposed by tbe senior author for the caudal and 

 cephalic portions of the ventral aspect of the prosencephalon and rhinencephalon, bounded 

 latero-dorsad by the F. postrhinalis and F. rhinalis respectively. They are of course con- 

 tinuous with each other. 



In the adult human brain these tracts are relatively so small and so obscured by the 

 overhanging convolutions of the hemisphere proper that they appear not to have been 

 defined. 



Tractus rhinalis, Tr. rA.— See § 1337. 



§ 1328. Trapezium, te.-Fig. 116 ; PI. I, Fig. 2 ; PI. 11, Fig. 3 ; §§ 1140, 1161, 1376. 

 Quain, A, II, 511. 



Syn. — Corpus trapezoides — Meynert (Strieker, A, 726) ; Huxley, A, 64. 



The quadrangular, slightly convex portion of the Area postpontilis in the angle formed 

 by the caudal margin of the pons and the lateral margin of the pyramis. It is sometimes 

 crossed by a faint cephalo-caudal band, as indicated on the right (left of the figure) side of 

 PI. II, Fig. 2. This band seems to be continuous with the Area otalis, and there is some- 

 times (as in Prep. 407, M. C. U.) an equally distinct band close to the pyramis and appar- 

 ently continuous with the Area eUiptica. 



Laterad, the trapezium is continuous with the Eminentia auditoria ; the N. abducens 

 arises between it and the pyramis, and the N. facialis between the pons and its latero- 

 cephalic angle. 



In man this area is covered by the greater caudal extension of the pons. 



