The Vascular System. 45 



imperfectly divided externally — the forward position of the first portion 

 of the aorta, and the position of the aorta as a median vertebral trunk. 



The vascular system is noteworthy for several departures from the 

 condition of symmetry, one of these having already been mentioned. In 

 addition, it is seen that in a mammal, as in terrestrial vertebrates generally, 

 the base of the pulmonary artery is rotated in a spiral fashion about 

 the base of the aorta, so that from its beginning on the right ventricle it 

 passes across the ventral surface of the base of the aorta, and divides on 

 the dorsal side of the latter into its two main branches. Moreover, the 

 separation of the ventricular portion of the heart into two chambers is 

 associated with an enormous increase in the muscularity of the wall in 

 the left ventricle, or, in other words, in that portion which is concerned 

 with the larger, systemic circulation. The inferior caval vein (Plate 

 VIII), a highly specialized vessel, is asymmetrical, since from its begin- 

 ning in the pelvic cavity to its termination on the right atrium it lies 

 wholly to the right of the median plane. The azygos vein of the thorax 

 (Plate VII), a vessel uniting the majority of the paired intercostal 

 veins, and interesting as a remnant of the primitive circulation, is also 

 asymmetrical, since the trunk lies to the right of the bodies of the 

 vertebrae, and is connected at its base with the right superior caval vein. 



In general, the blood which is distributed to the various parts of the 

 body passes through but one set of capillary vessels, and is then returned 

 through the systemic veins to the heart. In all vertebrates, however, a 

 special portion of the systemic venous circulation is set: aside as the 

 hepatic portal system, distinguished by the possession of a second series 

 of capillary vessels ramifying in the liver. Thus, in the rabbit, the blood 

 distributed to the stomach, spleen, and intestine through the coeliac, 

 superior and inferior mesenteric arteries, is collected into a main 

 intestinal vessel, the portal vein, and the latter, approaching the liver 

 through the lesser omentum, divides in that organ into a series of portal 

 capillaries. The portal capillaries, like the systemic capillaries proceed- 

 ing from the hepatic artery, unite in the tributaries of the hepatic veins. 

 In lower vertebrates, although not in the mammalia, a second system of 

 venous capillaries occurs in connection with the kidneys, and is known as 

 the renal portal system. 



The lymphatic system, both in its functional relation and in origin, 

 is an appendage of the venous portion of the vascular system. The 

 system is an important one, of which, unfortunately, little may be seen 

 by ordinary dissection, the structures which appear in this way being 

 the lymph glands, or lymph nodes, centres of cell formation, occurring 

 in the course of the conducting vessels. These as superficial structures 

 are found either singly, as in the head and neck, or more or less grouped, 

 as in the axillary and inguinal spaces. As deep structures they are 

 conspicuous in the intestinal mesenteries, and in the walls of the digestive 

 tube, occurring in the latter chiefly as continuous masses of lymph 

 follicles, as, for example, in the walls of the sacculus rotundus, the 

 vermiform process, or the tonsil ; or, again, as aggregated lymph follicles 

 (Peyer's patches) at various points in the intestinal wall. 



The conducting portion of the system comprises an extensive series 



