ACTION OF BLOOD-VESSELS IN NUTEITION. 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. 



Y2 



