80 GENERAL VIEW OF THE FUNCTIONS. 



surround them. In the higher Animals, the variety of tissues which present 

 themselves in the adult structure, all formed by a metamorphosis from the 

 original vesicles of the embryo, is very great ; but these are all nourished, in a 

 more or less energetic manner, by the blood conveyed to them in the network 

 of minute vessels which traverses them. Still between the reticulations of 

 these vessels, there must necessarily be islands of solid tissue (as seen in Fig. 

 90), of no inconsiderable size ; and the central portions of these must derive 

 their nourishment from the surrounding cells, exactly as in the humblest Cel- 

 lular Plants. Moreover there are some tissues in which, in the healthy state 

 at least, no very minute distribution of blood-vessels can be ascertained to 

 exist ; and in these the cellular nutrition must go on to a considerable extent. 

 The decay and renewal of such tissues, however, are by no means rapid ; and 

 it is only in such as require little change from time to time, and whose actions 

 are of a physical rather than of a vital character (such, for instance, as Car- 

 tilage), that this mode of nutrition is sufficient. 



89. In the nutrition of the tissues which are already completely formed, it 

 seems probable that the fluid portion of the blood performs the chief part. It 

 will be shown hereafter, that the particles of this substance, probably in virtue 

 of the preparation it has undergone by the agency of the cells just described 

 ( 86), which exist in blood as well as in chyle, and are known under the 

 name of colourless corpuscles, have the power of so arranging themselves as 

 to form a regular fibrous tissue. These are the only corpuscles existing in the 

 circulating fluid of Invertebrated animals ; but in the blood of Vertebrata there 

 are others, which contain a colouring fluid, and give to the whole mass its red 

 or purple hue. * These red corpuscles appear to serve the important purpose 

 of conveying oxygen from the lungs into the interior of the system, and of 

 carrying away carbonic acid from the tissues ; since it is evidently in them, 

 that the chief chemical changes effected by Respiration are produced ; and the 

 heat regularly maintained in any class of animals, bears a very close proportion 

 to the quantity of red particles in their blood. 



90. The history of the changes by which one group of cells is transformed 

 into bone, another into cartilage, another into nerve, another into muscle, and 

 so on, is extremely interesting, and will be given hereafter in as much de- 

 tail as the limits of this work permit. Of the reason why this variety of pro- 

 ducts should spring up, when the cells in which they all originate appear to 

 be so exactly alike, and have themselves a common origin, no account can 

 be given ; and this is one of the most cufious problems that at present offers 

 itself for investigation. The important^ discoveries, which are here briefly 

 summed up, are not confined to healthy structures ; for it has been ascertained 

 that diseased growths have a similar origin and mode of extension ; and that 

 the malignant character, assigned to Cancer, Fungus Hcmnatodes, and other 

 such productions, is to be traced to the fact, that they are composed of cells 

 which undergo little metamorphosis, and retain their reproductive power ;~-so 

 that from a single cell, as from that of a Vegetable Fungus, a large structure 

 may rapidly spring up, the removal of which is by no means attended with 

 any certainty that it will not speedily re-appear, from some germs left in the 

 system. 



91. The independent character of the cells in which all organized tissues 

 originate, might be of itself a satisfactory proof that, in Animals, as in Plants, 

 the actions of Nutrition are performed by the powers with which they are 

 individually endowed ; and that, whatever influence the nervous system may 

 have upon them, they are not in any way essentially dependent upon it. 

 Moreover, there is an evident improbability in the idea, "that any one of 

 the solid textures of the living body should have for its office, to give to any 

 other the power of taking on any vital actions ;" and the improbability becomes 



