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 very 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 three 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 
Transverse J 
Fic. 631.—Bratn or Horse; Dorsat View 
1, Entomarginal fissure; 2, marginal fissure; 
fissure 
, ABour } NATURAL SIZE. 
Olfactory bulb 
Frontal pole of hemisphere 
Cerebrum 
Occipital pole of 
hemisphere 
HARDENED in situ. 
3, ectomarginal fissure; 4, suprasylvian fissure. 
TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE 
BRAIN 
Primary SEGMENTS. SeconDARY SEGMENTS. DERIVATIVES. CavirTIes. 
Myelencephalon ......... Medulla oblongata........... 
Rhombencephalon (pos- (aPons | 
pera (coll aroeiaae 1 Metencephalon .......... { Cerebellum teh ae + Fourth ventricle 
ee ; atk § Anterior cerebellar peduncles | 
ba “ ( Isthmus rhombencephal . . | Anterior medullary velum 
Mesencephalon (middle . auadnpeing 
VESICIO) act oceeorncae Mesencephalon .......... { Sena Seance ages } Cerebral aqueduct 
Optie thalami Sis e85 Wiposkers : hird 
sterior part of thir 
Diencephalon............ ) Hypothalamic tegmenta. . > Se eaeaiet 
Prosencephalon (ante- ) Pineal body \ 
ior vesicle). ..... 1 ( Pituitary body bats eepete Anterior part of third 
fas } Optic nerves and retine...... ventricle. Lateral 
Telencephalon. ......++.- 1 ¢ | ventricles and olfac- 
Cerebral hemispheres 
Olfactory tracts and bulb. .... tory continuations. 
