ON THE INTERVERTEBRAL SUBSTANCE. 409 
body of the upper vertebre to slide upon the one below it (fig. 2), and 
so diminish the capacity of the spinal canal, as well as the general 
stability of the column. A forcible attempt to rotate the body upon 
the spine would, under similar conditions, be also attended by com- 
_ pression of the elastic pad, and considerable rotatory gliding of the 
vertebre. on one another (fig. 3). 
These difficulties are entirely surmounted by the existing mechan- 
ism (fig. 4), as may be most satisfactorily demonstrated by the em- Page 50. 
ployment of a model composed of two circular disks of wood bound 
together, with an interval between them, by tapes of similar lengths 
arranged obliquely and crossing one another, attached to opposite 
points on the margins of the disks. So connected, no gliding of any 
kind of the disks upon one another can be produced, and the only 
movements possible are their approximation either at all points, or at 
any part where compression is employed (figs. 5 and 6). 
It may not be out of place for me here to draw attention to one or 
two points which are associated with the erectness of the carriage of 
man, in contradistinction to the horizontal and oblique attitudes 
assumed by lower animals. 
The simple curve, concave ventrally, of the vertebral column of the 
higher Apes was most certainly shared by the human prdgenitor. In 
the young child it is found to exist. In its attempts to assume the 
‘upright carriage this progenitor must, equally certainly, have thrown 
the centre of gravity of its body directly above the hips, to do which 
it was necessary to bend the spine backwards. On account, however, 
of the thoracic region being rendered rigid by the attachment of its 
cage of ribs, and the sacrum being unmodifiable from its ankylosis, 
this flexion of the spine could only occur in the neck and loins; con- 
sequently the spinal flexures in man may be explained upon the 
assumption that the dorsal and sacral ventral concavities are the 
similar curves of the ancestral type, retained on account of the 
mechanical obstructions to their removal, whilst the ventral convexi- 
ties of the yielding cervical and lumbar regions are the means by 
which the centre of gravity in the erect position is carried to a point 
directly above the hip-joints. 
This assumption of a vertical attitude by a creature originally dif- _ 
ferentiated for a horizontal position of its body, has produced but 
marvellously slight inconvenience. If it had resulted in many, man ~ 
could scarcely have survived. There are one or two, however, which 
are most clearly traceable to this cause, including the painful tendency 
to prolapse, antiflexion, and retrofiexion of the uterus in women, as 
well as crural hernia in both sexes, and inguinal hernia in the male. 
In mammalian animals with the body horizontal the weight of the 
uterus is transmitted to the abdominal walls, at the same time that the 
