1881.] bou fWilder. 



Tr. (?^.— Tractus opticus. 8, 11, 14, 19. 

 Tr. prh. — Tractus postrhinalis. 3. 

 tz. — Trapezium. 2, 3. 



lil. — ^Velum (interpositum). ISTot shown. See p. 544. 

 vm. — ^Vermis (cerebelli). 1, 2, 15. 

 tiv. — Valvula (cerebelli). 4. 



A few of tlie terms included in the foregoing list need more extended 

 mention. 



The enceplialic segments. — It is often convenient, and always more philo- 

 sophical, to regard the brain as composed of a series of segments or dim- 

 sions, each consisting of a cavity, with its sides, floor, and roof variously 

 thickened, convoluted, or otherwise modified. So far as is known, the 

 anatomical divisions most conveniently made correspond essentially with 

 the series of embryonic vesicles. 



To these divisions, the German anatomists, following, I believe. Yon 

 Baer, apply the names vorderMrn, zwischenliirn, mitteUiirn, MnterMrn, and 

 nachhirn, which are commonly rendered in English by foreirnin, 'tween- 

 irain, midbrain, Mndbrain, and afterhrain. 



In converting these vernacular terms into technical, anatomists have 

 generally recognized the practical advantage of regarding the LoM olfac- 

 torii as a segment apart from the cerebral hemispheres, under the name of 

 rhinemcephalon. The hemispheres, including the striati, etc., constitute 

 the prosencephalon, and the Lobi optici, with the corresponding portion of 

 the Crura cerebri, form the mesencephalon. So far, all agree. But the 

 region including the thalami, between the prosen. and the mesen. has been 

 Tariously called deutencephalon, thalamencephalon, and diencephalon. Un- 

 able to ascertain which has priority, I select the last as the shortest and 

 most applicable. 



Upon the names of the remaining parts of the brain there is more 

 serious divergence of usage among tvriters. Owen (A, I, 293) calls it 

 all epencephalon ; but Huxley applies (A, 60) that name to the pons 

 and cerebellum as a division separate from the medulla, which he calls 

 myelencephalon, notwithstanding this term had been previously proposed by 

 Owen (A, I, 268) for the entire " cerebro-spinal axis." In this, Huxley is 

 followed by the English editor of "Gegenbaur" (A, xiii) notwithstand- 

 ing his admission that a diiferent nomenclature had been previously pub- 

 lished in Quain (A, II, 755). The editors of "Quain" recognize the two 

 divisions, and apply epencephalon to the pons and cerebellum, giving to 

 the medulla proper the name metencephalon. On all accounts, this seems 

 to me the best arrangement of terms for the encephalic segments, and is 

 followed in the present paper. 



TJie cmlice, or enceplialic "ventricles." — The incongruity of the anthropo- 

 tomical designations of the encephalic caA'ities has been pointed out by 

 Owen (A, I, 294, note), and the writer (9, 125). 



The canalis centralis expands into a cavity which, although the first of 



PROC. AMER. PHILOS. SOC. XIX. 109. oP. PRINTED DEC. 6, 1881. 



