390 On the Functions of progressive Motion 
along the whole length of the back. The peculiar mechanisnt 
by which the spine is rendered capable of answering a variety of 
Important purposes in the animal system, was fully explained ; 
and the several modifications of structure pointed out, which it 
receives in different tribes, in order to adapt it to the circum- 
stances in which they are placed, and to the various intentions of 
theirformation: ‘The system of organs by which the locometion 
of the body is effected, was next considered, in the relation which 
they bear to the element on which they are exerted. 
As aquatic animals present the simplest mechanical conditions 
with reference to locomotion, Dr. Roget began with the ex- 
amination of this function in fishes. He observed.that the buoy- 
ant force of the Suid which surrounded them, by counteracting 
nearly the whole of the force of gravjiy, superseded the necessity 
of limbs for the support of the body, which land animals require; 
and that the progress of a fish in the water is effected principally. 
by the muscular action of the tail, which, giving powerful lateral 
strokes, impels the animal forward on the same principle that 
a boat is moved in sculling. The modifying and regulating pow- 
ers of the fins were next explained, and elucidated by diagrams 
and drawings. The hydrostatic principles on which fishes are 
assisted in their ascent or descent in the water, by the dilatation 
or compression of the air-bladder, were stated, aud illustrated 
by some experlinents, in which similar effects were produced in 
glass vessels immerysed in water, but containing sufficient air to 
enable them to rise to the surface, or sink to the bottom, acecord- 
ing as the included air was made to expand or contract. The 
air bladder in fishes may be regarded as a refined apparatus in 
the body of these animals, expressly accommodated to the laws 
of hydrostatic pressure ; and as furnishing one out of the many 
instances that, exist, where philosophical principles have been 
applied, with manifest art and intention, for effecting a particu- 
lar purpose in the ceconomy. Those fishes which have no air- 
bladder, as flat fish, have this, want compensated by the great 
size and power of motion in the pectoral fins, which enable 
them to strike the water from above downwards with cousidera- 
bleforce. In the whale, and other animals of the cetaceous tribe, 
the body is rendered specifically lighter by the large quantity of 
oil which it contains, and which is especially accumulated about 
the head, as this part of the body is continually required to be 
raised above the surface, for the purpose of respiration. The 
various modes of progressive motion employed, by other aquatic 
tribes, both of reptiles and of mammalia, were also noticed. 
Dr. Roget next proceeded to consider the mechanism of land 
animals, beginning with serpents, and reptiles having short and 
imperfect feet. He showed the means by-which the former are. 
enabled 
