THE SPLEEN 377 



delicate strands into the gland substance. At the portal fissure it is abundant and 

 surrounds the vessels and ducts, which it accompanies in the portal canals of the 

 gland sul)stance. 



The gland substance is composed of the parenchyma and the interstitial 

 tissue. The parenchyma is made up of lobules, 1 to 2 mm. in diameter, which are 

 held together by a small amount of interlobular connective tissue. On account of 

 the very small amount of the latter, the lobulation of the horse's liver is not usually 

 at all distinct to the naked eye; for the same reason the organ is also quite 

 friable.' 



Vessels and Nerves. — Tlie portal vein enters at the jiortal fissure. It conveys 

 blood from tiie digestive tract and the spleen, which contains various products of 

 digestion and numerous white blood-cells. The hepatic artery also enters at the 

 portal fissure; it may be termed the nutrient vessel. All the blood is returned from 

 the liver to the posterior vena cava by the hepatic veins. The portal vein and the 

 hepatic artery both divide into interlobular branches, which run together in the 

 portal canals of the interlobular tissue. The branches of the portal vein (\\nia) 

 interlol)ulares) give off intralobular branches wliieh form i)lexuses of cajiillaries in 

 the lobules and give rise to a central vein (Vena centralis). The 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 - (X'eniv hepaticip) empty into tiie vena 

 cava as it lies in the fo.ssa of the gland. Their ultimate radicles are the central 

 lobular veins, which emerge from the bases of the lol)ules and join the sublobular 

 veins (Venae sul)lobulares) ; the latter unite to form the hepatic veins. The largest 

 hepatic veins, three or four in number, join the post(>rior vena cava just before it 

 leaves the liver to pa.ss through the diai:)hragni. 



The nerve-supply comes from the hepatic plexus, composed of branches from 

 the vagus and sympathetic nerves. 



THE SPLEEN 

 The spleen (Lien) is the largest of tlie ductless glands.^ It is situated chiefly 

 in the left parachondrium, 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 su])ject under different conditions, de- 

 pending chiefly on the great variability of the amount of blood contained in it. 

 The average weight is about 35 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. In the natural state it is soft and yielding, 

 but not friable. 



The weight appears to vary ordinarily from about one to eight pounds, although in large 

 horses the latter figure even may be exceeded without any apparent evidence of disease. There 

 does not seem to be any constant relation to the l)ody-weight. For example, the spleen of a 

 colt about ten months old weighed three and a half pounds, while it often weighs less than two 



1 In the young foal the interlobular tissue is more abundant and the lobulation correspond- 

 ingly distinct. 



- 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 which 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 epitheUal in origin 

 or structure, but is mesenchymatous. 



