THE NERVOUS SYSTEM 125 



portion of the rayelon 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 iippejr 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 Lenhossek on Mice, Guinea-pigs, Eabbits, 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 Eat, they run in the dorsal columns, in 

 the Eabbit, 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 likely 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 is unaccompanied 

 by exceptional conditions of other parts of the organism, it is 

 altogether improbable that it 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 



