449 THE NERVOUS SYSTEM. 
away from each other in an outward direction, and thus the ventral angle between 
them becomes greatly opened up. The growth which leads to the formation of the 
bulb appears, therefore, to take place chiefly on the ventral aspect of the vesicle. 
The dorsal wall remains thin and epithelial, and undergoes little or no development 
into nervous elements. 
The cavity of the original hind-brain is retained in the adult brain as the fourth 
ventricle ; and from what has been said aC ding the development of the different 
portions of the wall of the primitive hind-brain, it will be seen that in its lower or 
medullary part its dorsal wall or roof, to a large extent, remains epithelial. 
The parts of the adult brain which are derived from the rhombencephalon or 
hind-brain are those which he below the tentorium cerebelli in the posterior cranial 
fossa of the skull. 
Mesencephalon or Mid-brain—The mid-brain takes a much more prominent 
part in the formation of the early primitive brain than it does in the construction 
of the adult brain. It forms a very small part of the adult brain, and constitutes a 
stalk of connexion between the parts which are developed from the walls of the 
rhombencephalon and those which are developed from the walls of the prosence- 
phalon or fore-brain. ‘The entire wall of the mid-brain is transformed into nervous 
tissue. Thus, by the special development of the dorsal section of the wall, the 
corpora quadrigemina are formed. The lateral and ventral sections of the wall 
undergo a still more marked degree of growth-thickening, and the result is the 
formation of the two crura cerebri. The cavity of the mid-brain is retained as the 
harrow passage termed the aqueduct of Sylvius, which connects the third ventricle 
of the braim with the fourth ventricle. 
Prosencephalon or Fore-brain.—In its early condition one of the leading 
peculiarities of the fore-brain is its great width. It extends outwards on either 
side for a considerable dis- 
tance beyond the lateral 
walls of the mid - brain. 
These lateral expansions of 
the fore-brain are the optic 
vesicles, and at this stage 
“UY \SA\ sthey are in no way con- 
pos stricted off from the cen- 
OPTIC VES. q : 
optic STALK tral part of the cavity. 
Soon, however, the central 
portion of the fore-brain 
begins to expand upwards 
A. Through the fore-brain of the early human embryo. B. Through and forwards, whilst. the 
the fore-brain and optic vesicles of a Lepidosteus embryo of eight ee has ; F 1 
days (after Balfour and Parker, modified). terminal pol tions oO the 
optic vesicles likewise 
undergo enlargement; and the result is, that the originally single chamber shows 
subdivision into three parts, viz. a central portion or fore- brain proper, and two 
expanded optic vesicles, which are joined to the lower parts of the lateral 
aspects of the fore-brain proper by two short constricted tubular passages termed 
the optic stalks. 
The optic vesicle and the optic stalk become ultimately transformed into the 
retina of the eye-ball and the optic nerve. The series of changes which lead to this 
result are detailed in the section dealing with the anatomy of the organ of vision. 
The fore-brain undergoes a series ‘of remarkable developmental changes, the 
most striking of which is the formation of the cerebral hemispheres. The terminal 
or fore-portion of the fore-brain, in the first instance, expands in a forward and 
downward direction, and from the upper and lateral aspects of the new portion of 
the vesicle thus formed the cerebral hemispheres bulge outwards in the form of 
two hollow pouches. The hinder original part of the fore-brain is termed the 
thalmamencephalon or diencephalon, whilst the anterior part with the cerebral 
hemispheres, which protrude out from it, receives the name of telencephalon. 
The side walls of the diencephalon become thickened into the two large masses 
of gray matter termed the optic thalami; the floor or ventral wall develops into 
OPTIC VESICLE 
A B 
Fie. 328.—Two Cross SECTIONS THROUGH THE FORE-BRAIN. 
