intervertebral Substance in Fish and Quadrupeds. 183 
since in that state the central substance no longer supports 
the vertebras, and the joints must lose their proper firmness, 
which will be attended with many disadvantages. 
As the principal motive which induces me to prosecute the 
laborious researches of comparative anatomy, is to attain a 
more complete knowledge of the structure and functions of 
the human body, than can be acquired in any other way, and 
to apply that knowledge to the most useful of all purposes, the 
cure of diseases, the success which has attended my labours, 
in the present instance, affords me particular satisfaction ; it 
encourages me in the pursuit of those inquiries, and holds out 
an invitation to others, by showing them that the paths of 
nature, however frequently they have been traced, are not 
yet sufficiently explored. 
EXPLANATION OF THE PLATE. 
A longitudinal section of one of the intervertebral joints of 
the squalus maximus, after the fluid had been evacuated, and 
the parts had been steeped in water„ 
aaaa. The section of the vertebra to show its shape and 
the two concave surfaces which form the intervertebral cavi- 
ties. The vertebra itself is partly bone, and partly transparent 
cartilage ; the bony portion forms the two cup-like cavities, 
and the intermediate substance consists of bony cells in form 
of lozenges filled with cartilage. 
The cavity of the joint is in its contracted state, and the 
inner portion of the lateral ligaments, which is made up of thin 
layers of a loose texture, has its interstices loaded with water, 
