4j44 
The primary cephalic flexure is soon followed by the cervical flexure. 
Fic. 330.—PROFILE VIEW OF THE BRAIN OF A HUMAN 
THE NERVOUS SYSTEM. 
EMBRYO OF TEN WEEKS (His). 
The various cranial nerves are indicated by numerals. 
A, Cerebral diverticulum of pituitary body. 
diverticulum of pituitary body. 
B, Buceal 
occurs at the junction of the hind- 
brain with the spinal cord. Here 
the entire head is bent in a ventral 
direetion, and at the end of the fifth 
week the flexure is so pronounced 
that the cerebral and spinal cord 
portions of the neural tube meet each 
other at a right angle. In the later 
stages of development the cervical 
flexure becomes obliterated by the ele- 
vation of the head and the straighten- 
ing of the neck of the embryo. 
The third bend takes place in the 
region of the future pons Varolit 
(metencephalon), and is consequently 
termed the pontine flexure. It differs 
from the other flexures in being 
confined to the brain tube and in 
not in any way involving the entire 
head. Further, the bend is much 
more marked in the thick ventral 
wall than in the thin dorsal wall of 
the tube. The neural tube is 
doubled forwards on itself and 
the pons Varolii becomes developed 
in connexion with the summit of the 
curvature. In the further growth of 
the brain the pontine flexure becomes almost completely obliterated. 
By reason of these curvatures the early brain assumes a sinuous, zigzag or 
S-shaped outline 
when viewed from 
the side, and the 
relationship of its 
various parts be- 
comes materially 
altered. The 
essential factor 
at work in the 
production of the 
brain flexures 1s 
clearly the very 
unequal growth 
which takes place 
in different parts 
of the cerebral 
wall. 
The Basal and 
Alar Lamine of 
His.—It has been 
pointed out that, 
in the develop- 
ment of the spinal 
cord, each of the 
thick lateral walls 
of the neural tube 
is marked off into 
a dorsal or alar 
and a ventral or 
month. 
Lip o 
OPTIC \ 4 ‘<6 
VESICLE ) ja} 
I fn |=" 
a {=| > 
0 \e 
\ a \e 
Mw 
\ oO 
(SF 
“NG \ 
HEART \ 
CHORDA E 
DORSALIS LJ 
I 
A 
A409 AvNias 
Fic. 331.—DIAGRAMS TO ILLUSTRATE THE ALAR AND BasaL LAMINZ. 
cases the embryonic brain is represented in mesial section 
Hf 
sfeHlsil > 
HH r 
a PS > 
e|-|l/ A 
Ga | ie 
Sai 
se (ei z 
is = 
1s 
| > 
Sy} 
== 
In both 
(His). 
A. The different subdivisions of the brain are marked off from each other by dotted 
lines, and the dotted line running in the long axis of the neural tube indicates the 
separation of the alar from the basal lamina of the lateral wail. 
B. Mesial section through the brain of a human embryo at the end of the first 
Dotted lines mark off the different regions and also the alar and basal 
laminz from each other. 
H, Buceal part of pituitary body; RL, Olfactory lobe; C.Str, Corpus striatum ; 
A, Entrance to optie stalk ; O, Optic recess ; I, Infundibular recess ; T, Tuber 
cinereum ; M, Mammillary eminence. 
This — 
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