HEAD AND NECK OF THE HORSE 199 



hrachium,^ of which the anterior (bz'achium quadrigeminum anterius) 

 is scarcely demonstrable, since it becomes buried immediately on leaving 

 the colliculus. The brachium of the posterior colliculus (brachium 

 quadrigeminum posterius), on the contrary, is easily traced as a rounded 

 ridge running in a ventral and anterior direction to disappear under the 

 medial geniculate body. 



The Cerebral Peduncles (PeduucuW cerebri). — Each cerebral peduncle 

 forms the ventral part of its own half of the mid-brain, and is divisible 

 into two strata — the tegmentum ^ and the basis i:)edunculi. Of these, 

 the basis pedunculi is the smaller and the more ventral, and is separated 

 from its fellow by the interpedtincidar fossa (fossa interpeduncularis) in 

 the posterior part of which (substantia perforata posterior) there are 

 numerous small openings for the passage of arteries into the brain 

 tissue. The superficial distinction between the two parts of the 

 peduncle is indicated by two grooves. The lateral groove (sulcus 

 lateralis) is visible when the mid-brain is viewed from the lateral 

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

 extent of the basis pedunculi in a medial direction is marked by the 

 shallow groove from which the roots of the oculomotor nerve take their 

 superficial origin, the sulcus ncrvi oculomotoril. 



An indistinct strip of fibres winds round the cerebral peduncle 

 beginning at the brachium of the anterior colliculus and ending vaguely 

 about the mammillary body. This is the transverse peduncular 

 fasciculus (fasciculus pedancularis transversus). 



Dissection. — Cut across tlie mid-brain, making the section through 

 the anterior colliculus, but not so far forward as to injure the geniculate 

 bodies. 



An examination of the cut surface of the mid-brain reveals the 

 aqua'ductus cerebri (aqueduct of Sylvius), which represents the cavity 

 of the original embryonic vesicle from which the mid-brain develops, 

 and, in the adult, places the third and fourth ventricles in communica- 

 tion with each other. In general, the outline of the aqueduct in section 

 is lozenge-shaped, and around it is a collection of grey nerve matter 

 from which the fibres of the third and fourth and the mesencephalic 

 motor root of the fifth cerebral nerve take their real origin. 



The differentiation of the tegmentum and basis pedunculi can be 

 readily made by an observation of the position of the two grooves 

 already noted on the surface. In addition, a stratum of nerve tissue, 

 the siobstantia nigra, of somewhat darker colour, lies between the two 

 structures. 



1 Brachium [L.], ^paxii^" (brachion) [Gr.], an arm. 



2 Tegmeiitum [L.], a covering. 



