Their organs of digestion and nutrition are constructed according to a different type, and upon a more enlarged plan than in any of the classes enumerated in the preceding chapters; and parts are superadded to the digestive apparatus which in lower tribes had no existence. In addition to the usual subsidiaryglands, namely, the salivary and the hepatic, a third secretion is poured into the intestine along with the bile, derived from the pancreas, a viscus which we have not as yet met with. Throughout all the Mollusca we have found the bile secreted by the liver to be separated from arterial blood, as are the other secretions of the body; but in the Vertebrata it is from venous blood that the bile is formed, and in consequence an elaborate system of vessels is provided, distinct from the general circulation, by which a large supply of deoxygenized blood is conveyed to and distributed through the liver, constituting what is termed by anatomists the system of the vena portae: nay, more, in connexion with this arrangement we find another remarkable viscus make its appearance, the spleen, from which venous blood is copiously supplied to the portal vein, and added to that derived from other sources.

(1659). A still more important and interesting circumstance, which strikes the anatomist on comparing the Vertebrata with lower forms of existence, is the sudden appearance of an entirely new system of vessels, destined to absorb from the intestines the nutritious products of the digestive process, and to convey them, as well as fluids derived from other parts of the body, directly into the veins, there to be mixed with the mass of the circulating blood. These vessels, of which no traces have been detected in any of the Invertebrata, are called lymphatics and lacteals; but their structure and distribution will occupy our attention hereafter.

(1660). The blood of all the Vertebrata is red, and is composed of microscopic globules of variable form and dimensions in different animals. In the class of Fishes, owing to the as yet imperfect condition of the respiratory apparatus, the temperature of the body is scarcely higher than that of the surrounding medium; and even in Reptiles such is the languid condition of the circulation, and the incomplete manner in which the blood is exposed to the renovating influence of the oxygen derived from the atmosphere, that the standard of animal heat is still extremely low. But in the higher classes, the Birds and Mammalia, owing to the total separation of the systemic and pulmonary circulation, the effect of respiration is increased to the utmost; and, pure arterial blood being thus abundantly distributed through all parts, heat is more rapidly generated, the warmth of the body becomes considerably increased, and such animals are permanently maintained at an invariable temperature, considerably higher than that of the medium in which they live.

Hence the distinction generally made between the hot-blooded and cold-blooded Vertebrata.

(1661). The variations in the temperature of the blood, above alluded to, are, moreover, the cause of other important differences observable in the clothing, habits, and instincts of these creatures. To retain a high degree of animal heat necessarily requires a warm and thick covering of some non-conducting material; and consequently in the hair, wool, and feathers of the warm-blooded tribes we at once recognize the provision made by Nature for preventing an undue expenditure of the caloric generated in the body. Such investments, however, would be ill adapted to the inhabitants of a watery medium; and consequently the fish, destined to an aquatic life, and the amphibious reptile, doomed to frequent the mud and slime upon the shores, are deprived of such incumbrances, and clothed in a scaly or slippery covering more fitted to their habits, and equally in accordance with the diminished temperature of their blood.

(1662). Still more remarkable is the effect of a mere exaltation of animal heat upon the instincts and affections of the different races of the Vertebrata. The fishes, absolutely unable to assist in the maturation of their offspring, are content to cast their spawn into the water, and remain utterly careless of the progeny to be derived from it. The reptile, equally incapable of appreciating the pleasures connected with maternal care, is content to leave her eggs exposed to the genial warmth of the sun until the included young escape. But no sooner does the vital heat of the parent become sufficient for the purposes designed by Nature, than all the sympathies of parental fondness become developed, all the delights connected with paternity and maternity are superadded to other enjoyments; and the bird, as she patiently performs the business of incubation, or tenderly watches over her newly-hatched brood, derives a pleasure from the performance of the duties imposed upon her, second only to that enjoyed by the mammiferous mother, who from her own breast supplies the nutriment prepared for the support of her infant progeny.