436 OF NUTRITION. 



blood flowing through the vascular area ( 761). A similar purpose is proba- 

 bly answered by the transitory cells developed within the germinal vesicle, and 

 by those which appear at a similar period in the evolution of the descendants 

 of the "twin cells" produced in it. Many similar examples have been else- 

 where adduced.* We have thus a class of facts, which indicates that the con- 

 version of the Chemical compound into the Organizable principle the aplastic 

 into the plastic material is effected in the particular situations where it is most 

 wanted, by the vital agency of transitory cell-life ; that is, by the production of 

 cells which are not themselves destined to form an integral part of any, perma- 

 nent structure, but which, after attaining a certain maturity, reproduce them- 

 selves and disappear ; successive generations thus following one another until 

 the object is accomplished, after which they altogether vanish. We shall now 

 consider another class of facts, which seems to indicate that a change of this 

 kind is being continually effected in the nutritious fluids of Animals, during 

 their circulation through the body : by Cells, which are either carried about 

 with them, or which are developed for the purpose in particular situations, as 

 in plants. The former is the more common occurrence ; since the conditions of 

 animal life, usually involving a general movement of the body, require also a 

 general reparation of its parts, and an adaptation of the circulating fluid there- 

 fore to the wants of the whole fabric. 



579. It has been already shown, that Cells, which seem identical with the 

 white corpuscles of the Blood, are to be met with in the Chyle and Lymph, 

 fluids in which the elaboration of plastic fibrin is going on (564) ; and that 

 such an elaboration must be continually taking place in the blood itself, to 

 supply the plastic material which is being as continually drawn off by the 

 nutritive processes. Hence there would seem reason for attributing this im- 

 portant function to these floating cells ; the number of which present in the 

 fluids seems to bear a very close relation with the energy of this elaborating 

 process. It is a fact of great physiological interest and importance, that, whilst 

 the colourless corpuscles are to be met with in the nutritious fluids of all ani- 

 mals which possess a distinct -circulation, the red corpuscles are restricted to 

 the blood of Vertebrata. This observation, which was first put forth by Wag- 

 ner,t has been confirmed by the Author, who had been previously struck with 

 the very close analogy between the floating cells carried along in the current of the 

 circulation in some of the very transparent aquatic larvae (especially those of the 

 Culicidae), and the lymph-corpuscles of the frog. Now it is evident from this 

 fact, that, as the blood of Vertebrata is distinguished from their chyle chiefly by 

 the presence of red corpuscles in the former and by their absence in the latter, 

 the nutritious fluid of invertebrated animals is rather analogous (as Wagner has 

 remarked) to the Chyle and Lymph, than to the blood of Vertebrata. Or, to put 

 the same idea in another form, the presence of the colourless corpuscles in the 

 nutritious fluid appears to be the most general fact in regard to its character 



* There are probably cases, however, in which cells are very rapidly called into exist- 

 ence, without that preparatory elaboration of their nutrient materials, which we regard 

 as due to the vital operations of a preceding generation. Thus the Bovista giganteum, 

 a large fungus of the puff-ball tribe, has been known to increase, in a single fright, from 

 a mere point to the size of a huge gourd, estimated to contain 47,000,000,000 cellules. In 

 such a case it is difficult to suppose than any but the most rapid mode of generating cells 

 can have been in operation; and the idea that these could not have been developed by 

 any such elaborate process as that just alluded to, is borne out by the fact of their ex- 

 tremely transitory character, the decay of such a structure being almost as rapid as its 

 production. The same may be remarked of those fungous growths in the animal body, 

 which sprout forth most rapidly. Hence the apparent exception assists in proving the 

 rule. 



| Physiology, by Willis, Part ii. 



