410 ANATOMICAL TECHNOLOGY. 



the latter in the greater differentiation of the segments, and from both in the subglobular 

 form of the right hemisphere. 



All that is shown, however, might really exist in a brain without contravention of our 

 general knowledge of the structure and development of the organ ; hence, even if consid- 

 erable modification of detail should be required, such diagrams would still be useful as 

 an elementary introduction to the study of the brain. 



At the outset the student vrill do well to regard the diagrams as purely geometrical 

 figures or as representing hollow masses of wood or iron. The shading is conventional, 

 and intended to be uniform excepting with the three nerves of special sense in Fig. 110. 

 No attempt is made to indicate the difference between the true nervous parts and those 

 {eonarinm and hypophysis) which may consist of very different material, between the fibrous 

 and cellular portions of the nervous tissue, or between the longitudinal and transverse 

 fibers of the former. 



The names, however, are of course those which apply .to the various parts of the real 

 brain. They are usually written in full upon or near the parts, but in Pig. 113, A, and a 

 few other cases, abbreviations are employed upon one or both sides of the symmetrical 

 figures. 



The idea of diagrams like Fig. 110, 111, was derived from the very clear and suggestive 

 views of the typical brain given by Huxley (A, Fig. 19, 20) ; the only diafframs known to us 

 comparable with those in Fig. 113 were published by the senior author {22, PI. Ill), but 

 the membranes were not included. The transections of the frog's brain given by Stieda 

 (22, Taf. XVIII) are of actual sections and likewise omit the membranes. 



§ 1073. Fig. 110 — HoHzontnl Section. — This affords the most complete view of all of 

 the encephalic cavities (coeliae) and of their relations to each other and to the segments of 

 the organ. 



The coeliae form two series, mesal and lateral. The mesal coeliae are direct continua- 

 tions of the canalis centralis of the myelon, and open through the portae into the lateral 

 coeliae on either side. 



The walls of the coeliae are made continuous and of nearly uniform thickness ; in gen- 

 eral the two sides are similar, but those of the mesocoelia are made unlike so as to indicate 

 one of the most marked distinctions between the brains of the frog and Menobranchus, 

 which will be specified later. The left of the two lateral masses has approximately the 

 form of the hemisphere in the adult Amphibian ; the right has what may be regarded as 

 the primitive form of the hemisphere in the embryo in all Vertebrates ; compare Balfour, 

 A, II, Fig. 357. 



The pairs oi dotted lines at the left of the figure indicate the limits of the encephalic 

 segments, the names of which are written between the ends of the lines. Of these names, 

 only one, prosencephaion, is given in full ; to the abbreviations of the others, meten., epen., 

 etc., the word crphaloii is to be added in each case. On the right, the short straight lines 

 are likewise intersegmental like those in Owen (A, III, Fig. 46) ; but the longer lines (A-H'', 

 which pass to the similarly lettered parts of Fig. 113, are persegmental ; that is, they 

 pass through the several segments instead of between them. 



Three special sense nerves are represented. The auditorius {N. au.) arises from the 

 metencephalon ; the opticus (JV. op.) from the mesencephalon and diencephalon, and the 

 numerous olfactorii {WN. ol.) from the rhinencephalou. 



The only portion of the wall here particularly specified is the terma (lamina terminalis), 

 fonning the cephalic boundary of the aula, the last of the mesal series of cavities. 



§ 1073. Pig. Ill — Mesal aspect of the right half of an ideal simple brain, exclusive of 

 the rhinencephalou and the cephalic portion of the prosencephalon. — Of course only the 



