GENERAL OUTLINE OF DEVELOPMENT OF THE BRAIN. 441 
~ completely concealed by them, is the inter-brain or diencephalon. The principal 
_ parts forming this portion of the brain are two large masses of gray matter, termed 
the optic thalami. Between these is the third ventricle of the brain—a deep narrow 
cavity occupying the mesial plane. The third ventricle communicates with the 
lateral ventricles by two small apertures, called the foramina of Monro. 
The cerebrum is connected with the parts in the posterior cranial fossa (pons 
Varolii, cerebellum, and bulb) by a narrow stalk called the mid-brain, or 
mesencephalon. The mid-brain is built up of the crura cerebri, passing from the 
pons Varolii to the cerebrum; the corpora quadrigemina, forming its dorsal part ; 
the superior cerebellar peduncles, proceeding from the cerebellum to the cerebrum, 
ete. It is tunnelled by a narrow passage, the aqueduct of Sylvius, which extends 
between the fourth and third ventricles. 
' GENERAL OUTLINE OF THE DEVELOPMENT OF THE BRAIN. 
| The brain is developed from the expanded anterior portion of the primitive 
neural tube. In the section dealing with the general principles of Embryology it 
CERVICAL 
FLEXURE 
Ss 
aquoo avNid 
CEREBRAL 
HEMISPHERE 
PONTINE FLEXURE 
A B 
Fic. 327.—Two StTaGes IN THE DEVELOPMENT OF THE HuMmAN Brain (after His). 
A. Brain of an embryo of the third week. B. Brain of an embryo of five weeks. 
has been pointed out that this is marked off by two constrictions into three 
_ primitive cerebral vesicles, which are termed respectively the hind-brain or rhomb- 
encephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon. 
Hind-brain or Rhombencephalon.—The hind-brain is the largest of the three 
primary expansions of the neural tube: indeed, it may be said that in the earlier 
stages of brain development it is larger than both of the other primary subdivisions 
_ taken together. The portion immediately adjoining the mid-brain is constricted, 
and is termed the isthmus rhombencephali. This is a very small part, forming the 
extreme upper end of the vesicle, and from its walls are developed the superior 
cerebellar peduncles and a thin lamina, which is stretched across the middle line 
between them, called the valve of Vieussens or the superior medullary velum. 
Immediately behind the isthmus the hind-brain expands suddenly, and then slowly 
and gradually tapers as it passes downwards towards the spinal cord part of the 
neural tube. Its junction with the latter is very early indicated by a sharp bend 
in the tube, which is termed the cervical flexure. The large portion of hind-brain 
which extends from the isthmus to the cervical flexure is usually considered as 
being composed of two parts, viz. an upper portion, termed the metencephalon, and 
an inferior portion, called the myelencephalon. 
From the metencephalon are derived the cerebellum and pons Varolii. The 
cerebellum arises by a thickening of the dorsal wall of this portion of the vesicle, 
whilst the pons is derived from a thickening of the lateral and ventral walls. The 
myelencephalon gives origin to the bulb or medulla oblongata. This is chietly 
formed by a thickening of the lateral walls of this part of the vesicle. These fall 
