413 ANATOMICAL TECHNOLOGY. 



is here made black ; the shaded portions represent the lateral, dorsal and ventral columns 

 of alba. 



The abbreviations signify as follows:— CTra. d., Columna dorsalis — " posterior white" 

 colnmn. Clm.l., Columna lateralis — "lateral white" column. Glm.i:, Columna ven- 

 Iralis — "anterior white" column. F. dms., Fissura dorsimesalis — "posterior" fissure. 

 Jf. vms., Fissura ventrimesalis — "anterior" fissure. Crn. d., Cornu dorsale cinereae — 

 " posterior horn of gray matter." Grn. v., Cornu ventrale cinereae — " anterior horn of gray 

 matter." 



(B) Metencephalon. — The ventrimesal fissure is nearly obliterated, but the sides of 

 the dorsimesal are widely separated, and the central canal opened into connection with 

 the space, metaccelia, so formed. The roof of the metaccelia is the metatela, composed 

 of the pia and endyma, and the mesal ridge is indicated by the undulation of the latter. 



(C) Epencephalon. — The floor and sides of the epicoelia are similar to those of the 

 metaccelia in the frog and Menobranchus, but in Mammals the former is reinforced by the 

 pons, and the latter presents three sets of fibers, the pediineuli of the cerebellum. This 

 latter is here represented in its essential character as a bridge over the epicoilia (Ecker, B, 

 Abt. II, 8), without the mesal furrow which indicates the junction of the optici and 

 thalami of the two sides. 



(D) Mesencephalon. — Aside from its greater width, the chief difEerence between this 

 region and the epencephalon is the existence of the distinct dorsimesal furrow, whence 

 the name corpus iigeminum. In the higher Vertebrates, the floor of the mesocoelia is 

 more or less differentiated as the crura cerebri. In Menobranchus the mesocoelia is 

 large and simple ; in most Mammals it is narrowed by the approximately uniform thick- 

 ening of all the walls, and may be reduced to a mere passage. In the frog the cavity is 

 very irregular, and no attempt is here made to indicate its form, excepting on the left of 

 Fig. 110 ; hence the difference from Fig. 16 and 17 of Stieda (22). 



(E) Diencephalon. — This transection is through the caudal portion of the segment. 

 The thalami constitute the sides of the diacoelia, and their roof presents the special band 

 of fibers known as the postcommissura (pes.). At the ventrimeson the proper nervous 

 floor is absent, but the two membranes are unbroken. The slightly protuberant floor 

 answers to the more distinct tuber cinereum of the higher Vertebrates ; compare Stieda 

 (22, Fig. 18). In the right of the diaocelia is represented a section of the free part of 

 the right diaplexus of Menobranchus (§ 1097), which does not exist in the frog. 



(F) Diencephalon.— This is through the cephalic portion of the segment. The two 

 lateral figures represent the.caudal ends of the liemisphmrce, which project caudad consid- 

 erably beyond their points of attachment. In the diencephalic portion of the figure, the 

 sides are the thinner cephalic portions of the thalami, and the floor is reinforced by the 

 chiasma; the roof is membranous at the sides, but thickened at the meson to form the 

 conarium or its continuation {§ 1084). 



(G) Prosencephalon— .4 i<?a and Porta:— T\e two procoelice are seen to communicate 

 with a mesal cavity, the aula, through the two portae. The floor of the aula is here 

 formed by the terma, but the roof (aulatela) consists of only the membranes. Compare 

 Stieda (22, Fig. 21), where, however, the membranes are omitted, and the caudal border 

 of the prwcommissura is included, so as to separate the dorsal part of the aula proper from 

 the cephalo-ventral portion. 



(H) Prosencephalon— .HeTOwp^ffira.— This figure may represent the transection of 

 almost any portion of the prosencephalon or rhinencephalou cephalad of the terma ; com- 

 pare Stieda (22, Fig. 23). The hemispheres are usually in contact, but are united organ- 

 icaUy only in Mammals (by the callosum). Their cavities (proccelim) communicate with 



