VASCULAR SYSTEM. 125 



enormous number of red blood corpuscles, which are flat, disc-shaped 

 globules, contain the colouring matter (haemoglobin) and carry the 

 oxygen to the tissues. In addition to the red blood- corpuscles 

 there are small colourless cells in the blood the amoeboid white 

 blood-corpuscles (vol. i., fig. 19). 



Except in Amphioxus, in which the larger vascular trunks pulsate, 

 a definite part of the vascular system is always developed to form a 

 heart. The heart lies in the anterior part of the body cavity, and is 

 primitively placed exactly in the middle line. It has a conical shape 

 and is enclosed in a pericardium. The position of the principal 

 vessels and their connection with the heart are in the simplest case 

 as follows : A large artery the dorsal aorta runs along the verte- 

 bral column and gives off numerous lateral branches, corresponding 

 to the segmentation of the vertebral column, to the right and left. 

 Beneath this there is in the caudal region, an unpaired vein the 

 caudal vein, in the body cavity on the contrary a pair of veins 

 the inferior cardinal veins. These veins receive their blood from 

 lateral venous branches which proceed directly from the capillary 

 network of the arterial branches. Another principal vein the vena 

 cava inferior separated from the cardinal veins by the hepatic portal 

 system, and connected with two superior cardinal veins, conveys the 

 venous blood back to that portion of the heart which is known as the 

 auricle. From the auricle the blood flows into the the muscular ven- 

 tricle and is forced thence into an ascending artery (aorta ascendens 

 or cardiac aorta). The latter divides into lateral arterial arches which 

 pass towards the dorsal side and unite beneath the vertebral column 

 to form the anterior part of the dorsal aorta (aorta, descendens) 

 (vol. i., fig 57). 



This system of the aortic arches is, however, complicated in various 

 ways by the insertion of the respiratory organs in the course of the 

 circulation (compare vol. i., p.. 63 et seq). 



In all Vertebrates there is a system of lymphatic vessels. These 

 are a special part of the vascular system and contain a clear nutritive 

 fluid (chyle and lymph) which is filled with colourless corpuscles 

 (lymph corpuscles). They conduct the lymph (containing plastic 

 materials for the renewal of the parts of the blood which have been 

 consumed in metabolism) to the blood. The principal trunk of the 

 lymphatic system (the thoracic duct) runs alonfg the vertebral column 

 and in the higher Vertebrates opens into the upper part of the vena 

 cava superior. In the lower Vertebrates there are several communica- 

 tions between the lymphatic and vascular systems. Special gland- 



