220 



DISSECTION OF THE DOG 



since it becomes buried immediately on leaving the coUiculus. The 

 brachium of the posterior eoUiculus (brachium quadrigeminum posterius), on 

 the contrary, is easily traced as a rounded ridge-like eminence, running in 

 a ventral and nasal direction to disappear under an oval Avhite eminence, 

 the medial geniculate body. 



Corpora geniculata. — The geniculate bodies, medial and lateral, are 

 intimately connected with the thalamus and the corpora quadrigemina. The 

 lateral geniculate body (corpus geniculatum laterale) is closely applied to the 



Corpora quadrl<j<'.mina 



Corpus geniculatum 

 mediate 



.. Ccreietluin 



..,.'^' 'r. 



Medulla oblongata 



'br(s arcuaXce externx 



Tractits peduncularis trausversns ' 



Pedunculus cerebri 



n. trochlcnri 



Pons 



) Pyraniis 



I 

 Corpus trapezoideum 



Fig. 69. — Lateral aspect of hind-brain and mid-brain. 



posterior part of the thalamus, and from it the optic tract appears to arise. 

 The medial geniculate body (corpus geniculatum mediale) is an oval eminence 

 between the optic tract and the brachium of the posterior coUiculus. 



Tracttjs opticus. — The optic tract, in the form of a white band, curves 

 obliquely round the lateral part of the cerebral peduncle. To the naked eye 

 it appears to commence in the lateral and medial geniculate bodies. Micro- 

 scopically, however, its fibres are connected with the lateral geniculate body, 

 the anterior coUiculus and the thalamus. 



Peditnctjli cerebri. — Forming the ventral part of the mid-brain, each 

 cerebral peduncle is divisible into two portions — the tegmentum and the basis 

 pedunculi. Of these, the basis peduncuh is the smaller and the more ventral. 

 The superficial distinction between the two parts is indicated by two grooves. 

 The lateral groove (sulcus lateralis) is visible when the mid-brain is viewed from 

 the side, and runs from the pons to the medial geniculate body. The extent 

 of the basis pedunculi in a medial direction is marked by a groove from which 

 the roots of the oculomotor nerve take origin — the sulcus nervi oculomotorii. 



