IX VEINS; BLOOD 325 



veins other than those mentioned so, far. The numerous small veins 

 which collect the blood from the intestinal wall join together to form 

 larger and larger trunks until at last the whole blood-stream is collected 

 together in a single great vessel known as the hepatic portal vein, or 

 shortly the portal vein (Fig. 136, p.v). This passes forwards to the liver 

 where it breaks up into branches supplying the capillary network of that 

 organ. From this there pass away in turn efferent veins which eventually 

 join up to form a pair of hepatic veins opening into the sinus venosus. 

 It is thus clear that the blood collected from the intestine, which after 

 a meal is laden with absorbed food material, has, before it reaches the 

 heart, to pass through the capillary network of the liver, and it is while 

 it does so that the carbohydrate is extracted from it to be stored up 

 temporarily, as has already been mentioned, in the form of glycogen. 



Apart from the above-mentioned features of the venous system, 

 which are all of primary importance, a number of other details will be 

 gathered from an inspection of the diagram. In Scylliuni the anterior 

 cardinal, posterior cardinal and hepatic veins are greatly dilated and 

 are commonly spoken of as sinuses. In the case of the two last men- 

 tioned the right and left sinus are in contact with one another along 

 the mesial plane and their cavities communicate freely through gaps in 

 the intervening partition. In ScylUum the, anterior cardinal sinus extends 

 into the orbit so that the eyeball and its muscles are bathed with blood. 

 This orbital sinus communicates with its fellow by a narrow vein through 

 the floor of the skull (Fig. 133, posterior opening marked v). There opens 

 into the duct of Cuvier on each side a small " inferior jugular " vein 

 (i.j) which comes from the floor of the mouth along the outer side of the 

 pericardiac cavity. Into the posterior cardinal sinus there open (i) at its 

 extreme front end the subclavian vein (sc) from the region of the pectoral 

 fin, and (2) further back the lateral vein from the body-wall and (3) still 

 further back and ventrally the genital sinus from the ovary or testis. 



Within the system of vessels there circulates the blood. This con- 

 sists of a watery fluid (plasma) which carries in suspension cells belonging 

 to two very different types. The first of these (leucocytes, " white 

 corpuscles ") are small amoebocytes : the others (erythrocytes, " red 

 corpuscles ") are cells which have lost ■ their amoeboid character, the 

 cytoplasm having become stiffened into the form of a flattened elliptical 

 disc, and are impregnated with the remarkable iron-containing pigment 

 haemoglobin which gives the red colour to the blood. This substance 

 is remarkable above all for its peculiar affinity for oxygen. In the 

 presence of oxygen it immediately combines with it to form a com- 

 pound — oxyhaemoglobin — distinguished by its brighter red colour. The 



