THE SPLEEN 



439 



portal canals of the interlobular tissue. The branches of the portal vein (Vense 

 interlobulares) give off intralobular branches which form plexuses of capillaries in 

 the lobules and give rise to a central vein (Vena centralis). The interlobular 

 branches of the hepatic artery (Rami arteriosi interlobulares) are of relatively 

 small size. They supply mainly (if not exclusively) the interlobular tissue, the 

 capsule, and the walls of the vessels and ducts. The hepatic veins^ (Vena; hepatics) 

 empty into the posterior vena cava as it lies in the fossa of the gland. Their 

 ultimate radicles are the central lobular veins (Venae centrales), which emerge from 

 the bases of the lobules and join 

 the sublobular veins (Yenai sub- 

 lobulares) ; the latter unite to form 

 the hepatic veins. The largest 

 hepatic veins, three or four in 

 number, join the posterior vena 

 cava just before it leaves the liver 

 to pass through the diaphragm. 



The nerves come from the 

 hepatic plexus, which is composed 

 of branches from the vagus and 

 sjTnpathetic nerves. 



THE SPLEEN 



The spleen (Lien) is the 

 largest of the heterogeneous group 

 of organs which are usually desig- 

 nated ductless glands.^ It is situ- 

 ated chiefly in the left parachon- 

 driac region, in close relation to the 

 left part of the great curvature of 

 the stomach, to which its long axis 

 corresponds. Its size and weight 

 vary greatly in different subjects, 

 and also in the same subject under 

 different conditions, depending 

 chiefly on the great variability of 

 the amount of blood contained in 

 it. The average weight is about 35 

 to 40 ounces (ca. 1 kg.), its length 

 about 20 inches (ca. 50 cm.), and 

 its greatest width about 8 to 10 

 inches (ca. 20 to 25 cm.). It is 

 usually bluish-red or somewhat purple in color, 

 yielding, but not friable. 



The weight, which is extremely variable, appears to range ordinarily from about one to eight 

 pounds, although the latter figure even may be exceeded without any apparent evidence of disease. 



1 The hepatic veins may be recognized on section from the fact that they remain open, 

 being connected closely with the parenchyma. 



^ The ductless glands are organs which elaborate substances that pass directly into the 

 veins or lymphatics, instead of being conveyed away by ducts. This process is termed internal 

 secretion. The ductless glands include the lymph glands, which are described with the organs 

 of circulation; the thyroid and thymus bodies, described usually with the respiratory organs; 

 the adrenal or suprarenal bodies, described with the urinary organs; the pineal and pituitary 

 bodies, described with the brain; and the spleen, described with the digestive system as a matter 

 of convenience. The spleen is not, strictly speaking, a gland at all: it is not epithelial in origin 

 or structure, but is mesenchymatous. 



Fig. 375. — Spi.een of Horse; Parietal Surface. 

 a. Dorsal extremity or base; b, c, anterior and posterior basal 

 angles; d, anterior border; e, ventral extremity or apex; 1, 2, 3, 

 impressions of last three ribs. 



In the natural state it is soft and 



Cs '^ J o 



