620 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 circula- 

 tion, 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 portce : 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 YEKTEBEATA 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 INTERTEBRATA, are called lym- 

 phatics 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 in- 

 creased, 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 gene- 

 rated 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 



