214 THE CAT. [chap. vir. 



inter-osseous membrane. It then turns downvrards and passes to the 

 middle of the front of the ankle, where it assumes the name of 

 dorsal artery of the foot, and runs on between the metatarsals of the 

 index and middle digits, where it divides, one part sinking to 

 anastomose with the external plantar artery (as already mentioned), 

 and the other part going to the inner side of the foot. This and the 

 plantar arteries give off small vessels to the digits, similar to those 

 given off to the digits in the fore-paw. 



§ 22. The VEINS are, with the exception of the portal vein, 

 afferent vessels beginning by minute tubes, which converge and unite 

 to form larger and larger trunks. They cannot, like the arteries, 

 be grouped in only two sets of vessels — those of the pulmonary and 

 of the systemic circulation respectively — because there are veins 

 which belong to a third category, already noted as those of the 

 portal circulation, which ramify and distribute the venous blood 

 from the spleen, pancreas, stomach, and intestine, within the sub- 

 stance of the liver. "With the exception, however, of the veins of 

 the pulmonary and portal circulations, all the veins of the body are 

 larger or smaller tributaries to those great vessels, already more than 

 once referred to as the superior and inferior vense cava3, and which 

 open directly into the right auricle of the heart. The veins which 

 pass from the lungs to the left auricle, are those of the pulmonary 

 circulation; while those veins which arise in the spleen, pan- 

 creas, stomach, and intestine, and those which ramify within the 

 substance of the liver, together constitute the portal system. The 

 Avhole of the blood passes at each circuit through both the pulmonary 

 and systemic circulations, but only a portion of the blood has a share 

 each time in the portal circulation. Thus both the systemic and 

 pulmonary circulations are served both by arteries and by veins, 

 and in each the blood is in one part arterial and in another venous, 

 but the portal circulation is served by veins only, and its blood is 

 venous entirely throughout. 



There is yet another antithesis : in both the systemic and pul- 

 monary circulations the circuit is complete, the vessels being 

 connected together by capillaries, as well as by large trunks. The 

 portal circulation is incomplete in so far as it is connected by minute 

 vessels only, there being no direct connexion between the large trunks 

 Avhich send blood to, and those which receive it from, the liver. 



The ruLMONARY ^'I•:I^■s are the only veins which convey arterial 

 blood. They arise by minute branches in the lungs (as will be 

 again noticed with the description of the breathing organs) ; they 

 converge to form four vessels, two on each side in the root of each 

 lung. These empty themselves into the left auricle. Those of the 

 right side pass behind the i-ight pulmonary artery and on the dorsal 

 side of the vena cava superior and the aorta. Those of the left side, 

 which are the shorter, pass ventrally to the descending aorta. 



The pulmonary veins of the right side unite with one trunk just 

 before opening into the auricle. 



As has been before said, many of the systemic veins consist of a 



