260 DIGESTIVE SYSTEM. 



membrane lias the effect of being a distinct capsule : and (though 

 some have described a covering underneath, as separable from it, 

 which they have named the tunica cellulosa hepatis) anatomists 

 in general do not admit of any other tunic. 



Arteries. — The liver, like other true glands, is composed of 

 arteries, veins, excretory ducts, nerves, and absorbents, united 

 together by a particular tissue, to express which we have the 

 term, 'parenchijrna. Its artery, the hepatic, a branch of the ab- 

 dominal aorta, is but of small size in proportion to its bulk, and 

 in comparison with others which supply the viscera in the 

 vicinity : e. g. if we contrast the splenic artery with the spleen, 

 or the emulgent with the kidney, and then compare them with 

 the hepatic, and contrast the hepatic with the liver, we shall find 

 that the latter are remarkably disproportionately small. 



Vena Porta. — In the venous system of the liver we discover a 

 peculiarity, of which no parallel instance is to be found in the 

 animal structure : not only is it furnished with veins that per- 

 form the office of returning blood, but it has others for the pur- 

 pose of conveying blood to it, which are to be regarded in the 

 light of secerning arteries. The trunk they spring from is called 

 the vena porta, a vessel formed out of the union of the splenic 

 and mesenteric veins, which takes place immediately above the 

 pancreas. Its course lies over the duodenum, on the right of the 

 hepatic artery and duct, to the concave part of the liver: about 

 opposite to the middle of the right lobe, it bifurcates; the right 

 division directly enters the gland ; the left continues its course 

 forward, in company with the hepatic artery, and subdivides 

 into two others, which penetrate the middle portion and left lobe 

 of the organ. Their branches spread out in an arborescent man- 

 ner within the substance of the liver, ramify to great minuteness, 

 and at length radiate into a system of capillary tubes, which, 

 from some peculiarities they exhibit in their arrangement, have 

 been named penicilli. 



The hepatic veins, the vessels that return the blood conveyed 

 hither by the vena porlae and hepatic artery, are in the horse 

 remarkably small, but exceeding numerous : their orifices may be 

 seen, appearing like so many pin-holes, by slitting open the pos- 

 terior vena cava. 



The hepatic duct, remarkable for the whiteness of its coats, 

 will be found running along the upper and inner edge of the 

 right lobe, and receiving in its course many small ductiform tubes 

 from the interior of the gland : its trunk afterwards accompanies 

 the hepatic artery, to the right of which, and below the vena 

 portae, it continues its passage to the duodenum. This duct is a 

 muscular tube, having a membranous lining ; is large enough to 



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