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 mesen 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 Thoth of an inch ; but they vary from about Tenth to ghth. 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 dove 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 tluid 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 ag-ain 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 rneans 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 lytnph, there seems much reason to believe from. their similarity in size and ap pearance. Whilst the red corpuscles vary in dimension from less than iyouth of an inch (Musk Deer) to 5.1,th (Proteus), the colourless corpuscles have not been observed to depart widely from the diameter of Babuth of an 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 all 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 dose analogy between the floating cells carried along in the current of the circulation in some of the very transparent aquatic larv, (especially those of the Culicidw,) and the lymph-corpuscles of the Frog. Now it is evident from this fact, that, as the blood of Vertebrata is distingoished 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 function 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 Main malia, the white globules are nearly as nume rous as the red particles this, Mr. Gulliver has frequently noticed in fcetal deer of about un inch arid a half long. In a still smaller flatus the blood was pale from the preponderance of the white corpuscles. It is, therefore, a fact of much interest that, even in the mammiferous embryo, at the period when growth is most rapid, the circulating fluid has a strong analogy to that of the I nvertebrata. It then, too, bears in other respects the most striking analogy to chyle; since it consists of the fluid elaborated from the organizable matter supplied by the parent, and directly introduced into the current of the circulation. The function of the placental vessels may be regarded as double ; for they are at the same time the channel, through which the alimentary materials supplied by the pa rent are introduced into the circulating system of the fcetus, and the medium of aerating the fluid which has traversed the fcetal system. Ilence the placenta may be regarded as at once the digstive and the respiratory appa ratus of the fietus, and the fluid circulating through the cord as at once chyle and blood. lt is not until the pulmonary and lacteal vessels of the embryo have commenced their indepen dent operation, that the distinction between the blood and the chyle of the fcetus becomes evi dent; and we should expect, therefore, to find that the circulating fluid, up to the time of birth, contains a large proportion of white cor puscles, which is actually the case. There is a gradual decrease, however, in their proportional number, from the earlier to the later stages of embryonic life, in accordance with the dimi nishing energy of the formative processes. It has been also observed by Wagner* that the number of colourless corpuscles is always re markably great in the blood of well-fed frogs just caught in the summer season ; and that it is very small in those, which had been kept long without food, and in those examined during the winter.