THE NERVOUS SYSTEM 125 
portion of the myelon be considered as a predisposing factor in the 
degenerative processes apparent in such cases? A parallel to this occurs, 
it seems to me, in the processes of reduction at the upper part of the 
thorax already mentioned (ante, p. 43), and in the pathological processes 
which set in at the tips of the lungs, perhaps connected therewith. 
That there are also progressive processes going on in the 
human spinal cord is probable, from the following observations 
made by Lenhossék on Mice, Guinea-pigs, Rabbits, and Cats. In 
these animals the pyramidal tracts are much more feebly developed 
than in Man (in whom they attain their highest differentiation), 
and their position in the spinal cord varies greatly. In the 
Guinea-pig, Mouse, and Rat, they run in the dorsal columns, in 
the Rabbit, the Cat, and other Carnivora, in the lateral, and in 
Man, partly in the lateral and partly in the ventral columns. 
This may perhaps be indicative of a gradual shifting of these 
tracts from the dorsal to the ventral columns, as we pass from 
the lower to the higher Mammalia ; and it would be interesting to 
investigate this point in the Apes. Even in Man the definitive 
condition is not reached, for the fact that the pyramidal tracts 
may run either along the ventral or the lateral columns is 
evidence that they are still subject to variation. 
Since the pyramidal tracts cross one another completely 
in all animals which have been examined, it seems hkely that 
their alleged semi-decussation in Man is only apparent, as the 
elements of the ventral tracts do eventually cross one another. 
And further, since these ventral tracts are wanting in Man in 
fifteen cases per cent, it would be necessary, if belief in semi-decus- 
sation is to be persisted in, to consider that a certain number 
of individuals were remarkable exceptions in that important 
character. Inasmuch as this supposed variation 1s unaccompanied 
by exceptional conditions of other parts of the organism, it 1s 
altogether improbable that 16 exists. 
I must refer the reader to the works of Waldeyer for an 
account of the differences to be found between the human spinal 
cord and that of the Gorilla. 
Before turning to the condition of the brain, attention may 
be drawn to a small body which lies beneath the last coccygeal 
vertebra, known as the coccygeal gland. This, on account of its 
close relation to the arteria sacralis media, is usually, but, it 
seems to me, incorrectly, relegated in text-books of human 
anatomy to a connection with the vascular system. Considering 
the established fact that the caudal end of the spinal cord, at an 
