ACTION OF BLOOD-VESSELS IN NUTRITION. 323 
fluid into the neighbourhood of the part where it is to be 
employed; and the blood, or at least its organizable portion— 
the liquor sanguinis —must quit the vessels before it can be 
employed in the development of new tissue. We might illus- 
strate this by the distribution of water-pipes through a city; 
they might pass into every house, nay, into every room, and 
yet the water must be drawn from the pipes before it can be 
applied to any required purpose. The spaces untraversed by 
vessels have been shown to be larger in some tissues, and 
smaller in others; the distribution of the capillaries being 
more minute, in proportion as the nutritive actions of the 
part go on more energetically. Now in the embryo, even of 
the most complex and perfect animals, there is a period when 
no blood-vessels exist, the whole mass being made-up of cells, 
every one of which lives for itself and by itself, absorbing 
nutriment from a common source, and not at all dependent 
upon its brethren. It is only when a diversity of structure 
begins to show itself,—one part undergoing transformation 
into bone, another into muscle, and so on,—and when some 
portions of the fabric are cut-off from the direct supply of 
nourishment,—that vessels begin to show themselves. These 
are formed, like the ducts of Plants, by the breaking-down of 
the partitions between contiguous cells; they at first seem 
rather like passages or channels, than tubes with walls of their 
own; and this condition they retain in certain cases through 
life (§ 289). 
Repair of Injuries . 
389. Every animal possesses, in a greater or less degree, 
the power of not merely maintaining its organized fabric in 
its integrity, by the renewal of the parts which are from time 
to time passing into decay, but also of reproducing parts of 
that fabric which have been lost by disease or accident. This 
power seems greatest among the lowest tribes of Animals; in 
many of which the entire organism can be reproduced from a 
small portion of it, as is the case with the Hydra (§ 122), and 
with some species of Sea-Anemone (§ 126). In the Star-fish, 
a far more highly-organized animal, the regenerative power is 
more limited, though it is still very remarkably manifested; 
for if one, two, or more of the rays be broken or cut off, they 
are gradually restored, provided the central disc be uninjured, 
y 2 
