454 MANUAL OF ZOOLOGY. 



The fluid and originally soluble portions of the food, and the 

 chyle which is formed in the process of digestion, are taken into 

 the blood, the losses of which they serve to repair. Part of 

 the nutritive materials of the food is taken up directly by the 

 blood-vessels, and is conveyed by the "vena portae" to the 

 liver, whence it ultimately reaches the great veins which go to 

 the heart. The greater part, however, of the liquefied food, 

 constituting the chyle, is taken up, not by the blood-vessels, 

 but by a special set of tubes, which form a network in the 

 walls of the intestine, and are known as the "lacteals." In 

 these vessels, and in certain glands which are developed upon 

 them, the chyle undergoes still further elaboration, and is made 

 more similar in composition to the blood itself. All the lacteal 

 vessels ultimately unite into one or more large vessels which 

 open into one of the veins, so that all the chyle is thus finally 

 added to the mass of the circulating blood. 



The blood, then, or nutrient fluid from which the tissues are 

 built up, is formed in this way out of the materials which are 

 taken into the alimentary canal as food. In all the Vertebrata, 

 with the single exception of the Lancelet (Amphtoxus\ the 

 blood is of a red colour when viewed in mass. This is due 

 to the presence in it of an incredible number of microscopical 

 bodies, which are known as the " blood-corpuscles," the fluid 

 in which these float being itself colourless (fig. 245). 



In all the Vertebrata the blood is distributed through the 

 body by means of a system of closed tubes, which constitute 



Fig. 245. Blood-corpuscles of Vertebrata. a Red blood-discs of man ; b Blood- 

 discs of Goose ; c Crocodile ; d Frog ; e Skate. 



the " blood-vessels; " and in all exceptjheLajicelet, the means 

 of propulsion are derived from a 'CSnfractile muscular cavity 

 or' " heart," furnished with valvular apertures. In the most 

 complete form of circulation, as seen in Birds and Mammals, 

 the heart is essentially a double organ, composed of two 

 halves, each of which consists of two cavities, an auricle and 

 a ventricle. The right side of the heart is wholly concerned 

 with the "lesser" or pulmonary circulation, whilst the left 

 side is concerned with driving the blood to all parts of the 

 body (systemic circulation). The modifications of the circula- 

 tory process will be noticed in speaking of the different classes 



