REPARATIVE POWERS OF HIGHER ANIMALS. 
325 
requiring a higher degree of the stimulus of Heat, than does 
their ordinary nutrition. In Lizards , an imperfect reproduc¬ 
tion of the tail takes place when part of it has been broken 
off; but the newly-developed portion contains no perfect ver¬ 
tebrae, its centre being occupied by a cartilaginous column 
like that of the lowest fishes.—-In the warm-blooded Verte- 
brata generally, the power of reproduction after loss or in¬ 
jury seems much more limited. We do not find that entire 
parts or members once destroyed, are completely renewed; 
though very extensive breaches of substance are often filled 
up. The tissues most readily reproduced are Bone, the Simple 
Fibres (§ 22), and the Membranes (such as the Skin, the 
Mucous and Serous membranes), of which these tissues form 
the basis. As a general rule, losses of substance in Glandular 
tissue, Muscle, and other parts of comparatively high organi¬ 
zation, do not seem to be reproduced; but there is a curious- 
exception to this in the case of Nervous tissue, which, with 
Blood-vessels, is very readily re-formed in the new growths by 
which losses of substance are repaired, as we often see in the 
rapid skinning-over of a large superficial wound. One of the 
most remarkable manifestations of reparative power in the 
Human body, is the re-formation of an entire bone, when the 
original one has been destroyed by disease. The new bony 
matter is thrown-out, sometimes within and sometimes around 
the dead shaft; and when the latter has been removed, the 
new structure gradually assumes the regular form, and all the 
attachments of muscles, ligaments, &c., become as complete 
as before. A much greater variety and complexity of actions 
are involved in this process, than in the reproduction of whole 
parts in the simpler animals; though its effects do not appear 
so striking. It appears that, in some individuals, this regene¬ 
rating power is retained to a much greater degree than it is by 
the species at large; thus, there is a well-authenticated instance, 
in which a supernumerary thumb on a bo/s hand was twice re¬ 
produced, after having been removed from the joint. In many 
cases in which the crystalline lens of the eye has been re¬ 
moved, in the operation for cataract, it has been afterwards 
regenerated; and there is evidence that, during embryonic 
life, the regeneration of lost parts may take place in a degree 
to which we have scarcely any parallel after birth ; attempts 
being sometimes made at the re-formation of entire limbs, in 
