MEDULLA OBLONGATA OR BULB. 445 
basal lamina. This subdivision is also noticeable in the cerebral part of the neural 
tube, and the furrow on the inner aspect of the lateral wall, which indicates this 
subdivision, can be traced even in the adult brain throughout the greater part of 
_ its length. 
} In the spinal cord the motor cells are gathered in the basal lamina in one long 
~ continuous column. In the brain the corresponding cells from which the efferent 
fibres of the cranial nerves are given off are also placed within the basal lamina, 
_ but they are arranged differently. They no longer form a continuous column, but 
are collected together in disconnected clusters termed the motor nuclei, and they 
do not extend higher up than the mid-brain. No motor nuclei occur in the basal 
“lamina of the fore-brain, Indeed, the importance of the basal lamina diminishes 
as we pass from the lower to the higher parts of the brain. 
In the rhombic or hind-brain the ereater part of the medulla oblongata and of 
_ the pons Varolii is formed from the ‘basal lamin, whilst the cerebellum, with its 
: superior and inferior cerebellar peduncles, is derived from the alar lamine. In 
| the mid-brain the crura cerebri are the derivatives of the basal laminz, whilst the 
corpora quadrigemina are developed from the alar lamin. In the fore-brain the 
- subthalamic region and the optic vesicles are products of the growth of the basal 
~ laminee, whilst “the optic thalami and cerebral hemispheres spring from the alar 
lamin. 
The fact that the cerebellum and the cerebral hemispheres owe their origin 
- to the alar lamine is sufficient to show the predominant part which these lamine 
play in brain development, and the higher we ascend in the animal scale the more 
pronounced does this predominance become. 
: The following table gives a summary of the various developmental processes 
" which have been described in the foregoing pages :— 
; 
fe Bulb or medulla oblongata 
| Myelencephalon Lower part of the fourth ven- 
| tricle 
} Rhombencephalon ‘Cerebelltun 
or PAE Pons Varolii 
Hind-brain Mictencephalon (eee part of the fourth ven- 
| (posterior cerebral tricle 
vesicle) Isthmus rhombencephali | 
(narrow constricted part | Superior cerebellar peduncles 
immediately adjoiming | Valve of Vieussens 
the mesencephalon) 
Meare ee | Mesencephalon | Corpora quadrigemina 
or Mid-brain 
Encephalon | (middle cerebral [{ Bac Coie corey) 
; | a | Mid-brain | Aqueduet of Sylvius 
ol - vesicle) 
Brain Optic thalami 
Subthalamic tegmental regions 
ailaai ence hal Pituitary and pineal bodies 
Tha ree eee ! Structures in interpeduncular 
= 6 Space 
Prosencephalon Diencephalon Optie nerve and retina 
or Fore-brain Hinder part of the third ven- 
(anterior cerebral ) . tricle 
vesicle) ‘Cerebral hemispheres 
| Olfactory lobes 
Teleneon bale | Lateral ventricles 
ae ere Be Foramina of Monro 
Anterior portion of the third 
\ \ ventricle 
THE PARTS OF THE ENCEPHALON DERIVED FROM THE 
HIND-BRAIN. 
MEDULLA OBLONGATA OR BULB. 
The medulla oblongata or bulb is the continuation upwards of the spinal cord. 
It is not more than one inch in length, and it may be regarded as beginning at the 
