Images de page
PDF
ePub

According to Sherrington, an anterior cerebro-spinal tract in the spinal medulla of the anthropoid apes stands in connexion with the arm-centre in the cerebral cortex. If this is the case in man it must have other connexions as well, seeing that it is carried down the spinal medulla for a considerable distance beyond the level of the spinal segments which give motor fibres to the upper limb. In the lower apes an anterior cerebro-spinal tract Funiculus gracilis

does not seem to exist : Gracile nucleus

the whole pyramid Funiculus cuneatus

Cuneate nucleus

crosses over to the op

posite side of the spinal Tractus spinalis of

medulla in the shape trigeminal nerve of the lateral cerebro

spinal tract.
Nucleus tractus
--spinalis nervi
trigemini

As we have noted, the decussating pyramidal bundles pass through the anterior column of gray matter, and cut it into two portions (Figs.

489 and 490). The Lateral cerebrospinal tract basal part remains in

position on the anCentral gray matter

terior and Detached head of anterior

and lateral Central canal

column of gray matter

aspect of the central Fasciculus anterior proprius

Decussation of pyramids

canal, and forms part Pyramid Pyramid

of the thick layer of Fig. 489.-SECTION THROUGH THE INFERIOR END OF THE MEDULLA OBLONGATA

gray matter which OF A CHIMPANZEE.

surrounds it. The detached head of the anterior column is set free; and from the large multipolar cells which lie in its midst some of the fibres of the anterior root of the first cervical nerve, and also some of the root fibres of the accessory nerve, take origin.

As we proceed into the medulla oblongata another effect of the decussation
of the pyramids is seen in the submergence from the surface of the strand of
fibres which, in the anterior funiculus of the spinal medulla, lies to the lateral side
of the anterior cerebro-spinal tract, and which receives the name of the fasciculus
anterior proprius. While the decussation is going on the fasciculus proprius is
thrust aside, and in the medulla oblongata, it takes up its position as a flattened
band-like strand on
the lateral side of the

Funiculus gracilis
pyramid (Fig. 489).
Gracile nucleus

Funiculus

cuneatus When the decussation

Tractus spinalis is

of fifth nerve Cuneate completed, this nucleus

Nucleus strand is seen to lie

tractus spin.

alis nervi close to the median Gray matter

trigemini plane on the dorsal around canal

Fasciculus aspect of the pyramid, Central canal

spino

cerebellaris where it is separated

Detached head from its fellow of the

of anterior Decussation of

column opposite side by the

pyramids median raphe alone

Fasciculus

anterior proprius (Fig. 491). In the upper part of the

Fissura mediana anterior medulla oblongata it

Fig. 490.- TRANSVERSE SECTION THROUGH THE INFERIOR END OF THE approaches still nearer

MEDULLA OBLONGATA OF A FULL-TIME FETUS, to the dorsal surface Treated by the Weigert-Pal method. The gray matter is bleached white, and and appears to form

the medullated tracts of fibres are black. the greater part of a strand, which is termed the medial longitudinal bundle (Figs. 494 and 495). The detached head of the anterior column of gray matter of the spinal medulla, as it is traced upwards, is observed to cling closely to its original relationship with the fasciculus anterior proprius. It is applied to the lateral side of this strand, and, gradually becoming smaller, finally disappears at the level of the inferior part of the inferior olivary nucleus.

[graphic]
[graphic]

Cuneate and Gracile Fasciculi, with their Nuclei.—As the fasciculus gracilis and the fasciculus cuneatus of the posterior funiculus of the spinal medulla are traced up into the medulla oblongata they seem to increase in bulk, and in transverse sections they assume the form of massive wedge-shaped strands, quite distinct from each other. They increase in width and lose considerably in depth, and consequently the transverse diameter of the area which they occupy becomes greater. As a result of this, and also owing to the removal of the lateral cerebro-spinal tract from the lateral funiculus of the spinal medulla immediately in front, the posterior column of gray matter is gradually rotated forwards and comes to lie transversely and in the same straight line with its fellow of the

[merged small][graphic][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed]

Fig. 491.-SECTION THROUGH THE CLOSED PART OF THE HUMAN MEDULLA OBLONGATA IMMEDIATELY

ABOVE THE DECUSSATION OF TAE PYRAMIDS (Weigert-Pal specimen).

opposite side (Figs. 490 and 491). The substantia gelatinosa, at the same time, becomes increased in quantity and presents a horseshoe-shaped outline in transverse section. It clasps within its concavity the somewhat reduced head of the posterior column, and forms with it a conspicuous circular mass of gray matter which lies close to the surface, and produces upon it the bulging termed the tuberculum cinereum. The basal portion of the posterior column of gray matter remains upon the dorsal and lateral aspect of the central canal, and forms a portion of the central gray mass of the closed part of the medulla oblongata ; but very soon the neck of the column, which at this level is greatly reduced owing to the absence of entering posterior nerve-roots, is invaded by bundles of fibres which traverse it in different directions and convert it into a formatio reticularis. By this means the rounded head of the posterior column becomes cut off from the central gray matter, and from this point upwards it remains as an isolated gray column intimately associated with the spinal root of the trigeminal nerve. It has, in fact, become the nucleus tractus spinalis nervi trigemini.

The gracile and cuneate nuclei are seen in their most typical form in sections at the level of the decussation of the pyramids (Figs. 489 and 490). The gracile

According to Sherrington, an anterior cerebro-spinal tract in the spinal medulla of the
anthropoid apes stands in connexion with the arm-centre in the cerebral cortex. If this is
the case in man it must have other connexions as well, seeing that it is carried down the
spinal medulla for a considerable distance beyond the level of the spinal segments which
give motor fibres to the upper limb. In the lower apes an anterior cerebro-spinal tract
Funiculus gracilis
Gracile nucleus

does not seem to exist :

the whole pyramid Funiculus cuneatus

Cuneate nucleus

crosses over to the op

posite side of the spinal Tractus spinalis of

medulla in the shape trigeminal nerve of the lateral cerebro

spinal tract.
Nucleus tractus
--spinalis nervi
trigemini

As we have noted, the decussating pyra. midal bundles pass through the anterior column of gray matter, and cut it into two portions (Figs.

489 and 490). The Lateral cerebrospinal tract basal part remains in

position on the anCentral gray matter

terior and lateral

Detached head of anterior
Central canal

column of gray matter

aspect of the central Fasciculus anterior proprius

Decussation of pyramids

canal, and forms part Pyramid Pyramid

of the thick layer of Fig. 489.-SECTION THROUGH THE INFERIOR END OF THE MEDULLA OBLONGATA OF A CHIMPANZEE.

gray matter which surrounds it.

The detached head of the anterior column is set free; and from the large multipolar cells which lie in its midst some of the fibres of the anterior root of the first cervical nerve, and also some of the root fibres of the accessory nerve, take origin.

As we proceed into the medulla oblongata another effect of the decussation
of the pyramids is seen in the submergence from the surface of the strand of
fibres which, in the anterior funiculus of the spinal medulla, lies to the lateral side
of the anterior cerebro-spinal tract, and which receives the name of the fasciculus
anterior proprius. While the decussation is going on the fasciculus proprius is
thrust aside, and in the medulla oblongata, it takes up its position as a flattened
band - like strand on
the lateral side of the

Funiculus gracilis
pyramid (Fig. 489).
Gracile nucleus

Funiculus
When the decussation

cuneatus

Tractus spinalis is completed, this Cuneate

of fifth nerve nucleus

Nucleus strand is seen to lie

tractus spinclose to the median Gray matter

trigemini plane on the dorsal around canal

Fasciculus aspect of the pyramid, Central canal

spino

cerebellaris where it is separated

Detached head from its fellow of the

Decussation of
opposite side by the

column
pyramids
median raphe alone

Fascicultus

anterior proprius (Fig. 491) In the upper part of the

Fissura mediana anterior medulla oblongata it

FIG. 490.-TRANSVERSE SECTION THROUGH THE INFERIOR END OF THE approaches still nearer

MEDULLA OBLOSONTA OP A FULL-TIME FETUS, to the dorsal surface Treated by the Weigert-pal

Toery matter is bleached white, and and appears to form

the

nre black. the greater part of a strand, which is termed

[graphic]
[graphic]

as it is traced upwards, is observed to cling closely to its original relationship with the fasciculus anterior proprius. It is applied to the lateral side of this strand, and, gradually becoming smaller, finally disappears at the level of the inferior part of the inferior olivary nucleus.

Cuneate and Gracile Fasciculi, with their Nuclei.—As the fasciculus gracilis and the fasciculus cuneatus of the posterior funiculus of the spinal medulla are traced up into the medulla oblongata they seem to increase in bulk, and in transverse sections they assume the form of massive wedge-shaped strands, quite distinct from each other. They increase in width and lose considerably in depth, and consequently the transverse diameter of the area which they occupy becomes greater. As a result of this, and also owing to the removal of the lateral cerebro-spinal tract from the lateral funiculus of the spinal medulla immediately in front, the posterior column of gray matter is gradually rotated forwards and comes to lie transversely and in the same straight line with its fellow of the

[merged small][graphic][subsumed][merged small][subsumed][merged small][subsumed][merged small][subsumed][subsumed][merged small]

Fig. 491.-SECTION THROUGH THE CLOSED PART OF THE HUMAN MEDULLA OBLONGATA IMMEDIATELY

ABOVE THE DECUSSATION OF TAE PYRAMIDS (Weigert-Pal specimen).

opposite side (Figs. 490 and 491). The substantia gelatinosa, at the same time,
becomes increased in quantity and presents a horseshoe-shaped outline in trans-
verse section. It clasps within its concavity the somewhat reduced head of the
posterior column, and forms with it a conspicuous circular mass of gray matter
which lies close to the surface, and produces upon it the bulging termed the
tuberculum cinereum. The basal portion of the posterior column of gray matter
remains upon the dorsal and lateral aspect of the central canal, and forms a
portion of the central gray mass of the closed part of the medulla oblongata; but
very soon the neck of the column, which at this level is greatly reduced owing to
the absence of entering posterior nerve-roots, is invaded by bundles of fibres
which traverse it in different directions and convert it into a formatio reticu?
By this means the rounded head of the posterior column becomes cut off f
central grav matter, and from this point upwards it remains as an isol
tely associated with the spinal root of the trigeminal nerve

tractus spinalis nervi trigemini.
"Zuclei are seen in their most typical form in

n of the pyramids (Figs. 489 and 490). Th

Gracile nucleus

nucleus appears in the form of a relatively slender mass of gray matter in the interior of the funiculus gracilis.

The cuneate nucleus is a direct offshoot from that part of the base of the posterior column of gray matter which is preserved as a portion of the central gray mass.

In transverse section it is seen to invade the funiculus cuneatus upon its deep aspect, and it gradually grows backwards into its substance. It presents

a very different appearance from Cuneate nucleus

the gracile nucleus, because

throughout its whole length the Cuneate nucleus gray nucleus and the fibres of Tractus spinalis of fifth nerve

the strand are separated from Nucleus tractus spinalis n. trigemini

each other by a sharp line of Fasciculus spino. demarcation. A little higher up cerebellaris Lateral cerebro-spinal a second and much smaller mass tract

of gray matter appears in the Detached anterior funiculus cuneatus, superficial to column of gray matter

the main nucleus. This is termed Decussation of pyramids

the accessory or the lateral cuneate nucleus (Fig. 491).

As a series of sections is

studied from below upwards, it Fig. 492.-SECTION THROUGH THE INFERIOR PART OF THE

will be noticed that the number MEDULLA OBLONGATA OF THE ORANG.

of fibres in the gracile and cuneate funiculi rapidly decreases, until, at the level of the clava and cuneate tubercles, it is seen that these eminences are composed almost entirely of the gray nuclei, covered by a thin layer of the few remaining fibres of the two fasciculi involved. It would appear that no fibres belonging to the funiculus gracilis and funiculus cuneatus get beyond these nuclei. They all end in fine terminal ramifications around the cells of the nuclei. In the case of the funiculus cuneatus the bundles of fibres, as they pass from the surface into the subjacent gray nucleus, are very distinctly seen in transverse sections through the bulb.

When the medulla oblongata opens up into the fourth ventricle the gracile and cuneate nuclei are pushed laterally by the expanding ventricular floor, and the gracile nucleus soon comes to an end; but the cuneate nucleus extends upwards for a short distance farther, and Funiculus gracilis

Gracile nucleus terminates only Funiculus

Cuneate when the restiform

nucleus body begins to take

spinalis

of trigeminal definite shape on its

Nucleus tractus lateral aspect.

spinalis nervi

trigemini Decussatio

Decussation of Lemniscorum. Spino-cere.

- lemniscus

medialis Immediately above the level of the de

Medial

accessory cussation of the

olivary nucleus pyramids another

Fila of
Inferior end

hypoglossal decussation of fibres of olive takes place in the

Pyramid substance of the medulla oblongata in the median plane, Fig. 493. TRANSVERSE SECTION THROUGH THE CLOSED PART OF A FETAL and upon the dorsal MEDI'LLA, IMMEDIATELY ABOVE THE DECUSSATION OF TAE PYRAMIDS. aspect of the pyra

Treated by Weigert-Pal method. mids. This is the decussation of the lemniscus medialis, or the sensory decussation, so-called in contradistinction to the term “ motor decussation,” which is sometimes applied to the decussation of the pyramids. The fibres which take part in this decussation are called internal arcuate fibres, and they are derived from the cells of the gracile and cuneate nuclei. From the deep aspects of these nuclei these fibres

[graphic]
[graphic]
« PrécédentContinuer »