780 NERVOUS SYSTEM OF THE HORSE 



contain vascular convolutions and are designated the median and lateral chorioid 

 plexuses of the fourth ventricle (Plexus chorioides ventriculi quarti). They appear 

 to lie within the ventricle, but are really excluded from the cavity by the epithelial 

 lining, which they invaginate. 



THE MESENCEPHALON 



The mesencephalon or mid-brain connects the rhombencephalon with the 

 fore-brain. In the undissected brain it is covered dorsally by the cerebral hemi- 

 spheres. It consists of a dorsal part, the corpora quadrigemina, and a larger 

 ventral part, the cerebral peduncles, which are visible on the base of the brain. 

 It is traversed longitudinally by a narrow canal, the cerebral aqueduct, which con- 

 nects the fourth ventricle with the third (Fig. 639). 



The corpora quadrigemina^ are four rounded eminences which lie under the 

 posterior part of the cerebral hemispheres. They consist of two pairs, separated 

 by a transverse groove. The anterior pair (Colliculi nasales) are larger and much 

 higher than the posterior pair. They are gray in color, almost 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 behind by a transverse 

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

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

 band of white matter termed the posterior peduncle (Brachium aborale).^ 



The cerebral peduncles (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 disappearance 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 superficial origin of the oculomotor nerve. 

 The triangular depression between the diverging peduncles is the interpeduncular 

 fossa (Fossa interpeduncularis) . It is covered to a large extent by the hjrpophysis 

 cerebri or pituitary body, a discoid brown mass which is connected with the base of 

 the brain by a hollow stalk, the infimdibulum. The posterior part of the fossa is 

 pierced by numerous minute openings which transmit blood-vessels, and is therefore 

 termed the substantia perforata posterior. The objects here belong to the dien- 

 cephalon, and will be described later. The lateral aspect of the peduncle is marked 

 by a groove (Sulcus lateralis mesencephali) which indicates 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 fibers, 

 passing dorsally and backward to the anterior cerebellar peduncle; these belong 

 to the fillet 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 (Aquseductus cerebri)* is the canal which extends 

 through the mid-brain from the third to the fourth ventricle. It is largest beneath 

 the posterior pair of corpora quadrigemina. It is surrounded by a layer of gray mat- 

 ter (Stratum griseum centrale), in the ventral part of which are the nuclei of origin 



' In the new nomenclature the term lamina quadrigemina is applied to the dorsal mass of 

 the mid-brain, and the four eminences which it bears are the corpora quadrigemina. 



2 In man a distinct superior brachium connects the superior pair with the lateral geniculate 

 body, but in the domesticated animals the union with the optic thalamus is too direct to allow of 

 any definite arm being recognized. 



' Also termed the crura cerebri. 



* Also termed the aqueduct of Sylvius. 



