664 NERVOUS SYSTEM OF THE HORSE 



higher than the posterior pair. They are gray in color, ahnost hemispherical, and 

 are separated by a narrow furrow which leads forward to the subpineal fovea. 

 A wide groove intervenes between them and the optic thalami. The posterior pair 

 (Colliculi caudales) are relatively small and are paler than the anterior pair. They 

 are marked by a wide median depression, and are limited below by a transverse 

 furrow (Sulcus postquadrigeminus), at either side of which the trochlear (fourth) 

 nerve emerges. Laterally each is prolonged to the inner geniculate body by a 

 band of white matter termed the inferior brachium.^ 



The cerebral peduncles or crura cerebri (Pedunculi cerebri) appear on the 

 base of the brain as two large rope-like stalks which emerge from the pons close 

 together and diverge as they extend forward to enter the cerebrum. At the point 

 of disai)pearance the optic tract winds obliquely across the peduncle. About half 

 an inch further back a small tract (Tractus peduncularis transversus) curves across 

 the peduncles, and behind this near the median line is the sui)erficial origin of the 

 oculomotor (third) nerve. The triangular depression between the diverging 

 peduncles is the interpeduncular space (Fossa interpeduncularis). It is covered 

 to a large extent ])y the pituitary body, a discoid brown mass which is connected 

 with the base of the brain by a hollow stalk, the infundibulum. The posterior 

 part of the space is pierced by numerous minute openings which transmit blood- 

 vessels, and is therefore termed the locus perforatus posticus. The objects in the 

 space belong to the diencephalon, and will be described later. The lateral aspect 

 of the peduncle is marked by a groove (Sulcus lateralis mesencephali) which indi- 

 cates the division into a dorsal part, the tegmentum, and a ventral part the basis 

 pedunculi; these are separated by a layer of dark gray matter, the substantia 

 nigra. The triangular area (Trigonum lemnisci) above the lateral groove is faintly 

 marked by fillers passing obliquely upward and backward to the anterior cerebellar 

 peduncle; these belong to the fiUet or lemniscus, an important tract that connects 

 the thalamus and corpora quadrigemina with the sensory reception nuclei of the 

 opposite side of the medulla. 



The aqueduct of the cerebrum or of Sylvius (Aquaeductus cerebri) is the canal 

 which extends through the mid-brain from the fourth to the third ventricle. It is 

 surrounded by a layer of gray matter (Stratum griseum centrale), in the ventral 

 part of which are the nuclei of origin of the oculomotor and trochlear nerves, and 

 laterally nuclei of the mesencephalic roots of the trigeminal nerves. 



THE DIENCEPHALON 



The diencephalon or inter-brain comprises the thalamus and a number of 

 other structures grouped about the third ventricle, the cavity of this division of the 

 l)rain.- To expose its dorsal aspect, the greater part of the cerebral hemispheres, 

 the corpus callosum, the fornix, the hippocampus, and the tela chorioidea of 

 the third ventricle must be removed. 



The thalamus (or oi)tic thalamus) is the ]irinci])al liody in this part of the brain. 

 It is a huge ovoid gray mass placed obliquely across the dorsal face of each 

 cerebral peduncle, so that the long axes of the two thalami would meet in front 

 about at a right angle. Medially they are fused to a large extent, and around the 

 area of adhesion they are separated ]\y a sagittal circular space, the third ventricle. 

 The dorsal surface is conv(>x in both directions, and is separated from the overlying 

 hii)pocami)us by the tela ciiorioidea (or velum interpositum). Laterally it is sep- 



' In iiiaii a distinct .superior hrachiuiii cotinofts Mio superior pair with the lateral geniculate 

 body. l)Ut 111 the domcstirated animals tiie union with the optic thalamus is too direct to allow of 

 any ilefimte arm heing recoiinized. 



; On a strictly emhryolofricaj basis the optic part of the hypothalamus, comprising the 

 anterior i)art of the tlnrd \-entricle and the structures associated with it, belong to the telen- 

 cephalon, but will be considered here as a matter of convenience. 



