454 THE NERVOUS SYSTEM. . 
tract in the lateral column of the cord is seen to become looser in its formation ; 
then coarse strands leave it, pass right through the anterior horn of gray matter, 
and, crossing the mesial plane, take up their position in the other side of the 
medulla, close to the antero-median fissure. Strands from the right crossed 
pyramidal tract alternate with corresponding strands from the left side, and the 
interval between the bottom of the antero-median furrow and the gray matter 
surrounding the central canal becomes filled up with a great mass of intercrossing 
bundles of fibres. When the decussation is completed the pyramid is seen to be 
composed of a solid and compact bundle of fibres, well marked off from the 
surrounding structures, which hes at the side of the antero-median fissure of the 
medulla. 
As a rule the inner three-fourths of the pyramid is composed of fibres which, lower 
down in the opposite lateral column of the cord, form the crossed pyramidal tract, whilst 
the outer fourth of the pyramid proceeds downwards in the anterior column. of the cord of 
the same side as the direct pyramidal tract. A considerable amount of variation, however, 
occurs in the proportion of fibres which is allotted to the formation of these two tracts of 
the cord. Sometimes the crossed pyramidal tract is much larger than usual, and then the 
direct pyramidal tract suffers a corresponding diminution in size. Cases indeed occur in 
which the entire pyramid enters into the decussation, and in these there is no direct 
pyramidal tract in the cord. Further, it is not uncommon to meet with variations of an 
opposite kind which lead to an increase of the direct pyramidal tract at the expense of the 
crossed tract. In the majority of cases the decussation appears to be symmetrical—the 
division of the pyramid at the lower end of the medulla being into parts of corresponding 
size on the two sides ; in certain instances, however, the decussation is asymmetrical, and 
the corresponding pyramidal tracts on opposite sides of the cord are then unequal in size. 
Seeing that the direct pyramidal tracts undergo a gradual decussation in the anterior 
commissure, as they descend in the cord, the final resuliil is the same, no matter what varia- 
tions oceur in the decussation at the lower part of the medulla. 
The variations indicated above receive an additional interest when viewed in the light 
of comparative anatomy. It would appear that only in man and the anthropoid apes is 
the decussation of the pyramids in the lower part of the medulla incomplete. According 
to Sherrington, a direct pyramidal tract in the cord of the anthropoid apes stands in 
connexion with the arm-centre in the cerebral cortex. In the lower apes a direct 
pyramidal tract does not seem to exist: the whole pyramid crosses over to the opposite 
side of the cord in the shape of the crossed pyramidal tract. 
As we have noted, the decussating pyramidal bundles pass through the anterior 
horn of gray matter of the cord, and cut it into two portions (Fig. 336, B). The 
basal part remains in position on the anterior and lateral aspect of the central 
canal, and forms part 
“Funiculus gracilis of the thick layer of 
Funiculus eray matter which 
cuneatus 
Gracile nucleus 
sui aneotor surrounds © it. The 
Cuneate fifth nerve detached head of the 
nucleus Substantia anterior horn 18 set 
ie sy Tee ip: Zz : gelatinosa i 7 
Giay matters ie : ) Upp = =e) Roland free; and from the 
round canal |g ee large multipolar cells 
rec 7 . . . . 
Central canal r7-cerebellar which he in its midst | 
piace some of the fibres of — 
Detached head Ay oe 
ofanterior the anterior root of 
cornu the first cervical nerve, 
Anterior basis- and also some of the _ 
bundle ' fp : 
root fibres ot thespinal 
accessory nerve, take 
origin. 
On proceeding up 
Treated by the Pal-Weigert method. The gray matter is bleached white, and into the amedulla a 
the medullated tracts of fibres are black. other effect of the 
decussation of the 
pyramids is seen in the submergence from the surface of the strand of fibres which, 
in the anterior column of the cord, lies to the outer side of the direct pyramidal 
Decussation of 
pyramids 
Antero-median furrow 
Fic. 337.—TRANSVERSE SECTION THROUGH LOWER END OF THE MEDULLA 
OF A FULL-TIME Fa@Tus, 
