HEAD AND NECK OF THE HORSE 197 



consists of a pale and relatively small central part (homologous with the 

 posterior lobe of the human hypophysis), connected with the brain by 

 the infundibulum, and a much larger and darker glandular part 

 (anterior lobe), developed originally from the upper part of the 

 primitive buccal cavity. 



The connections of the lamina terminalis have already been 

 noted (page 188). 



The third ventricle (Ventriculus tertius). — The third ventricle 

 is the laterally restricted space between the two tlialami. The choroid 

 tela forms its roof, while its floor corresponds to the optic chiasma, tuber 

 cinereum, mammillary body, posterior perforated substance and the 

 tegmenta. Anteriorly the cavity is bounded by the lamina terminalis, 

 the columns of the fornix and the anterior commissure. It com- 

 municates with the lateral ventricles through the inter- ventricular 

 foramen of Monro, and posteriorly an aditus ^ (aditus ad aquasductum 

 cerebri) gives access to the aqueduct of Sylvius. 



The thick intermediate mass, passing across the ventricle from 

 thalamus to thalamus, reduces the cavity to an annular space, the 

 regularity of the contour of which is broken by several recesses. The 

 recess of the infundibulum (recessus infundibuli) extends into the 

 hypophysis. The optic recess (recessus opticus) is just dorsal to the 

 optic chiasma. In the base of the pineal body is a small pineal recess 

 (recessus pinealis), and above it is a much more extensive suprapineal 

 recess (recessus suprapinealis). 



Associated with the third ventricle are the anterior and posterior 

 commissures. The anterior commissure (commissura anterior) is a 

 transverse band of fibres, largely concerned in connecting the olfactory 

 and piriform lobes of opposite sides, readily demonstrable in the hemi- 

 sected brain, where it appears as a small, oval, white object immediately 

 in front of the columns of the fornix. In the specimen from which 

 the structures overlying the thalami have been removed, a glimpse of 

 the commissure may be obtained between the slightly diverging 

 columns of the fornix. The posterior commissure (commissura 

 posterior) consists of a small white strand crossing the brain at the 

 base of the pineal body and dorsal to the entrance to the aqueduct. It 

 is best demonstrated in a median longitudinal section of the brain 

 The connections of its fibres are still obscure. 



Dissection. — It is now possible to make a complete examination of 

 the mid-brain. Strip tlie membranes from the corpora quadrigemina that 

 lie immediately behind the thalami, and from the adjacent part of 

 the cerebellum. Be careful to preserve the small trochlear nerve that 



1 Aditus [L.], an approach, an access, an entrance. 



