180 Mr. Home on the Nature of the 
skate is found by Mr. W. Brande to have the same properties, 
as far as the small quantity that can be collected admits of 
examination, with that in the squalus maximus. 
Although this structure of the intervertebral joint appears 
to be common to fish in general ; the form of the cavity is 
not in all exactly the same ; in the skate it is very similar 
to that in the squali, but in the common eel, it is more ob- 
long, the longitudinal diameter being about one third longer 
than the transverse one. 
It is evidently contrived for producing the quick vibratory 
lateral motion, which is peculiar to the back bones of fish 
while swimming, and enables them to continue that motion 
for a great length of time, with a small degree of muscular 
action. 
In the sturgeon, there are some curious peculiarities in the 
structure of the spine. Externally there is the common appear- 
ance of regular vertebrae, but these prove to be only cartilagi- 
nous rings, the edges of which are nearly in contact, and are 
united together by elastic ligaments, forming a tube the whole 
length of the spine, this is lined throughout its internal sur- 
face with a firm compact elastic substance, about the thick- 
ness of the cartilaginous tube, within this is a soft flexible 
substance in a small degree elastic ; in the centre there is a 
chain of cavities in the form of lozenges, containing a fluid, 
and communicating with one another by very small apertures 
bearing a slight similarity to the intervertebral cavities of the 
spine in other fish. 
As all the different parts of which this spine is composed 
are more or less elastic, except the central fluid, it must have 
great flexibility adapting it to the motions of this particular 
