HEAD AND NECK OF THE HORSE 



177 



white object that subsequent dissection will show to be connected with 

 the columns of the fornix. The tuber cinereurn is a less prominent 

 grey elevation, between the mammillary body and the optic chiasma, 

 connected by a hollow stalk, the infundibuhtm, with the hyjjophysis} 



On a level with the interpeduncular fossa, the cerebral hemisphere is 

 raised into the piriform lobe, a small anterior continuation of which is 

 separated from the main mass by a deep transverse depression occupied 

 by the middle cerebral artex-y. In front of the depression is the 

 olfactory'-^ lobe, consisting of the olfactory bulb, from which arises a 

 flattened white band, the olfactory tract. At its posterior end the 

 tract divides into medial and lateral strioi^ the former disappearing 

 into the fissure between the two cerebral hemispheres, the latter 

 coursing, as a white band, across the surface of the anterior prolonga- 



Metathalamus. 

 Epithalamus._ 



Diencephalon. y 



Thalamus, v 



Mesencephalon. 

 1 



._ Corpora quadrigemina. 

 V-- Isthmus rhombencephali. 



- Metencephalon. 

 - Cerebellum. 



_ Myelencephalon. 



Telencephalon. - 



Brachium conjunctivum and 

 velum medullare anterius. 



Pallium. I I , 



Rhinencephalon. i ' 



Corpus striatum, i 



Entrance to optic stalk. 



Pars optica hypothalami 

 Pars mamillaris hypothalami 



Medulla oblongata. 



Pons. 



Pedunculus cerebri. 

 Fig. 81. — Diagram of the alar and basal lamince of the embryonic brain. 



tion of the piriform lobe, and finally disappearing in the prominent part 

 of this lobe. Between the two striae is a grey eminence, the olfactory 

 trigone, the greater part of which is pitted with holes, thus forming the 

 so-called anterior perforated substance. 



Development of the Brain. — The whole of the central nervous system — spinal 

 cord and brain — develops from an embryonic neural tube, the right and left halves 

 of which are divisible into a ventral zone or basal lamina, and a dorsal zone or alar 

 lamina, united by a dorsal roof plate and a ventral ^ooj- jjZa^e. The development ot 

 the spinal cord is comparatively simple, since it consists essentially of a gradually 

 increasing thickening of the right and left walls of the neural tube by the formation 

 of nervous tissue, the original lumen of the tube remaining as the definitive central 

 canal of the spinal cord. 



The development of the brain is, naturally, more complicated. The anterior 



1 vTTOfpvaii (hypophysis) [Gr.], an undergrowth. 

 - Olfacio [L.], to smell. ^ Stria [L.], a furrow, groove, channel. 



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