486 VERTEBRATA. 



vertebrate division of the animal world, which must not be omitted 

 in this preparatory survey of their organization. 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 chapter ; and parts are superadded to the digestive 

 apparatus which in lower tribes had no existence. In addition to 

 the usual subsidiary glands, 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 con- 

 veyed to and distributed through the liver, constituting what is 

 termed by anatomists the system of the vena portte : 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. 



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 sys- 

 tem 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 INVER- 

 TEBRATA, ara called lymphatics and lacteals, but their structure 

 and distribution will occupy our attention hereafter. 



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 con- 

 dition 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 slow. But in the higher 

 classes, the Birds and Mammalia, owing to the total separation of 



