NUTRITION. 



745 



the production of cells which are not them- 

 selves destined to form an integral part of any 

 permanent structure, but which, after attaining 

 a certain maturity, reproduce themselves 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 seem 

 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 pur- 

 pose 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 

 constant general reparation of its parts, and an 

 adaptation of the circulating fluid therefore to 

 the wants of the whole fabric. 



In the chyle drawn from the lacteals near 

 the intestinal tube, there is but little fibrin ; and 

 very few of the peculiar chyle-corpuscles are 

 seen in the fluid. In the chyle of the meseu- 

 teric glands, on the other hand, the corpuscles 

 are extremely numerous ; and they are always 

 readily seen in the chyle of the central lac- 

 teals, receptaculum chyli, and thoracic duct, 

 though their number is considerably less than 

 in chyle drawn by pricking the lacteals of the 

 mesenteric glands. The average size of these 

 corpuscles is about ^th of an inch ; but they 

 vary from about ^th to as ^th. The smallest 

 are usually found in the peripheral lacteals; 

 the largest in the thoracic duct. They are 

 evidently cells in process of developement ; and 

 from the appearances presented by those which 

 are seen in the chyle of the thoracic duct, there 

 can be little doubt that they have the power of 

 reproducing themselves in the ordinary mode. 

 The first appearance of these cells in large 

 number is exactly coincident with the first 

 appearance of fibrin in the chyle, at least to 

 an amount sufficient to produce spontaneous 

 coagulation; and the delay of the chyle in the 

 mesenteric glands appears to aid in their deve- 

 lopement, and to assist their operation. In the 

 lower Vertebrata the absorbent system has none 

 of these (so called) glands ; and hence we see 

 that they are not essential to the performance of 

 its functions. But in such animals the vessels 

 are immensely extended in length ; whilst in 

 the warm-blooded Vertebrata, in whose con- 

 formation the principle of concentration ope- 

 rates to the greatest possible extent, we see no 

 such prolongation ; the end being answered by 

 the excessive convolution of the absorbents in 

 the mesenteric glands, where it seems probable 

 that the chyle is delayed during the develope- 

 ment of its characteristic cells. Similar state- 

 ments apply to the lymph, and to the lymphatic 

 vessels and glands. This fluid is probably to 

 be regarded, not as a product of the decompo- 

 sition of the tissues, which is destined to be 

 thrown out of the system, but as the product 

 of that secondary digestion, by which a portion 

 of the materials that have formed a component 

 part of the tissues, and have been set free by 

 their disintegration, is again rendered subser- 



vient to nutrition, and reconveyed into the 

 current of the circulation. Into the arguments 

 in favour of this view (which differs from the 

 ordinary doctrine regarding the function of 

 the lymphatic system) we cannot here enter ; 

 but it may be remarked that the animal matter 

 of the lymph is mainly of an albuminous cha- 

 racter, and that it gradually undergoes a trans- 

 formation into fibrin during its passage through 

 the absorbent vessels and glands. 



The continuation of this process in the blood 

 is believed by the writer to be effected by 

 means of the white, or colourless, corpuscles, 

 to which increased attention has lately been 

 directed. That these are identical with, or are 

 the immediate offspring of, the corpuscles of 

 the chyle and lymph, there seems much reason 

 to believe from their similarity in size and ap- 

 pearance. Whilst the red corpuscles vary in 

 dimension from less than -R^th of an inch 

 (Musk Deer) to 3: j 7 th (Proteus), the colourless 

 corpuscles have not been observed to depart 

 widely from the diameter of 35 Sjo tri of ^n inch in 

 any vertebrated animal ; consequently, while 

 they are but little larger than the average of red 

 corpuscles in man, and are scarcely distinguish- 

 able from them, except by the practised micro- 

 scopist, they are far more minute than the oval 

 blood-discs of reptiles and fishes, and are at 

 once recognised, even by a cursory observer. 

 Now it is a fact of great physiological interest 

 and importance, that whilst the colourless cor- 

 puscles are to be met with in the nutritious 

 fluids of ull animals, which possess a distinct 

 circulation, the red corpuscles are restricted to 

 the blood of Vertebrata. This observation, 

 which was first put forth by Wagner,* has been 

 confirmed by the writer of this article, 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 larva?, (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 solely 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 fluids appears to 

 be the most general fact in regard to its cha- 

 racter throughout the whole animal scale ; 

 whilst the presence of red corpuscles in that 

 fluid is limited to the vertebrated classes. 

 Hence it would not be wrong to infer that the 

 Junction of the colourless corpuscles must be 

 of a general character, and intimately connected 

 with the nutritious properties of the circulating 

 fluid ; whilst the function of the red corpuscles 

 must be of a limited character, being only re- 

 quired in one division of the animal kingdom. 

 Further, it has been noticed by Mr. Gulliver 

 that in the very young embryo of the Mam- 

 malia, the white globules are nearly as nume- 



* Physiology, by Willis, Part II. 



