446 THE NERVOUS SYSTEM. 
decussation of the pyramidal tracts, which takes place about the level of the 
foramen magnum. From this it proceeds upwards in a very nearly vertical direction, 
and ends at the lower border of the pons Varoli. At first its girth is similar to 
that of the cord, but it rapidly expands as it approaches the pons, and consequently 
it presents a more or less conical form. Its ventral surface lies behind the grooved 
surface of the basilar portion of the occipital bone, whilst its dorsal surface is sunk 
into the vallecula of 
the cerebellum. The 
medulla oblongata is 
a bilateral structure, 
and this is indicated 
; : Optic nerve 
Optic chiasma 
Optic eens eee i 
Corpus geniculatum 
externum 7/7 
- « | 
Corpus geniculatum Ss 
—Infundibulum 
——Tuber cinereum 
ale 
Corpora mammillaria 
Oculo-motor nerve (LIL.) 
internum 
Locus perforatus 
Trochlear nerve (1V.) 
winding round the crus 
on the surface by a 
continuation upwards 
Bosricus Ye cerebri of the antero-median 
Uy and postero - median 
Wi ‘Trigeminal nerve (V.) fissures of the cord 
Middle peduncle Ng j bh 
of the cerebellun #2 Abducent nerve(VI) on the ventral and 
EE ee) dorsal aspects of the 
Chey oN Auditory nerve (VIIL.) medulla 
ifor popes Se | it a. 
Restiform body- 1 |i _Vago-glossopharyngeal The antero-median 
Olive | nerve (LX. and X.) 
= 
groove (fissura medi- 
ana anterior), as it 
passes from the cord 
on to the medulla, is 
interrupted at the 
level of the foramen 
magnum by several 
strands of fibres, 
whichcross the mesial 
plane from one side 
to the other. This intercrossing is termed the decussation of the pyramids. Above 
this level the furrow is carried upwards to the lower border of the pons. Here it 
expands slightly and ends in a blind pit, which receives the name of the foramen 
cecum of Vicq d’Azyr. The postero-median fissure (fissura mediana posterior) is only 
carried up on the lower half of the medulla. As it ascends it rapidly becomes 
shallower, and, halfway up, the central canal of the cord opens on the dorsal surface 
of the medulla. At this poimt the lips of the postero-median fissure are thrust 
apart from each other and constitute the boundaries of a triangular field, which is 
thus opened up on the dorsal aspect of the medulla. This triangular field is the 
lower part of the fossa rhomboidalis, or the floor of the fourth ventricle of the brain. 
The lower half of the medulla, containing as it does the continuation of the central 
canal of the cord, is frequently termed the closed part of the medulla; the upper 
half, above the opening of the canal, which by its dorsal surface forms the lower 
part of the floor of the fourth ventricle, is often called the open part of the 
medulla. 
Deferring for the present the examination of the medullary part of the floor 
of the fourth ventricle, the appearance presented by the surface of each side of 
the medulla, from the antero-median fissure in front to the postero-median fissure 
and the lateral limit of the floor of the fourth ventricle behind, may now engage 
our attention. In the spinal cord the corresponding surface area is divided into 
three districts or columns by the emerging motor roots and the entering sensory 
roots of the spinal nerves. Of these the latter enter along the bottom of the 
postero-lateral groove, whilst the motor fascicles are spread over a relatively broad 
surface area and have no groove in connexion with their emergence from the 
cord. In the case of the medulla corresponding rows of nerve-fascicles enter and 
emerge from the surface of each side. The efferent fascicles are the root-bundles 
of the hypoglossal nerve, and they carry up the line of the anterior nerve-roots of 
the cord. In one respect, however, they differ; they emerge in linear order and 
along the bottom of a distinct furrow, termed the antero-lateral furrow, which 
"Hy poglossal 
Pyramid— \ neve (XII) 
Anterior superficial___-.-_—_— F¥S=— 
arcuate fibres } 31)i 
Spinal accessory 
nerve (XI.) 
Decussation Oj ea 
pyramids 
First cervical nerve 
Fic. 332.— Front View OF THE MEDULLA, PONS, AND MESENCEPHALON OF A 
FULL-TIME HUMAN F@TUSs. 
