772 



NERVOUS SYSTEM OF THE HORSE 



various parts of the tube. In the higher animals the result is that 'the tubular character of the 

 brain is not ^'ery evident, since the lumen comes to consist of four irregular cavities, the cerebral 

 ventricles, which are connected by narrow passages. The tube is first subdivided by two con- 

 strictions into tliree brain vesicles, termed respectively the hind-brain or rhombencephalon, the 

 mid-brain or mesencephalon, and the fore-brain or prosencephalon. The hind-brain gives rise to 

 three secondary segments and the fore-brain to two. The annexed table indicates the origin of the 

 principal structures of the fully developed brain from the primitive vesicles. It has become 

 customary to describe the brain with reference to its embryological relations. 



Longitudinal 

 fissure 



Transverse fissure 



Olfactory btilb^ 



Frontal pole of hemisphere 



Cerebrum 



y Occipital pole of 



hemisphere 



Cerebellum 



Medulla, oblongata 



Fig. 631. — Brain of Horse; Dorsal View, About 7 Natural Size. Hardened in situ. 

 1, Entomarginal fissure; 2, marginal fissure; 3, ectomarginal fissure; 4> suprasylvian fissure. 



TABLE INDICATING THE DERIVATION OF THE PRINCIPAL 



BRAIN 



Phimart Segments. Secon'dary Segments. Derivatives. 



Myelencephalon Medulla oblongata 



I Pons 



\ Cerebellum, 



Rhombencephalon (pi 



terior vesiclej J INIetencephalon 



PARTS OF THE 



Cavities. 



- Fourth ventricle 



Mesencephalon (middle 



[ Isthmus rhombencephali . 



f Anterior cerebellar peduncles. 

 \ Anterior medullary velum . . , 



eaeucepuaiou trniuuie / ,^i j ■ - \ 



--de) Mesencephalon { g^Xd redunde?"'. :;:;;::} Cerebral aqueduct 



Prosencephalon 

 rior vesicle) . . 



(ante- 



Optic thalami 



Diencephalon -j Hypothalamic tegmenta . . 



Pineal body 



Pituitary body 



Optic nerves and retinae. . 

 Cerebral hemispheres . . , . 

 Olfactory tracts and bulb. 



Telencephalon. 



Posterior part, of third 

 ventricle 



Anterior part of third 

 ventricle. Lateral 

 ventricles and olfac- 

 tory continuations. 



