514 OF SECRETION. 



tion.* In the Human species the gall-bladder is rarely absent, except in cases of 

 malformation depending upon general arrest of development, in which several 

 organs are involved. The Excretory Ducts of the Liver and Gall-bladder have 

 three coats, an internal or mucous, a middle or fibrous, and an external or 

 areolar. The internal coat is continuous with the Mucous membrane of the 

 Intestinal tube, into which it opens ; and the whole glandular structure may 

 indeed be considered as a complex prolongation of this, copiously supplied 

 with blood-vessels, and packed into the smallest possible compass. The mid- 

 dle or fibrous coat bears a considerable resemblance in aspect to that of the 

 Arteries ; in its properties, however, it is still more nearly allied to true muscle, 

 being capable of exhibiting contraction on the application of stimuli to the gan- 

 glionic nerves supplying it ( 200) ; and in some instances of obstruction, it has 

 presented an appearance very closely resembling that of the muscular coat of 

 the alimentary canal.t Dr. Davy has pointed out that the mucous coat of the 

 Ductus communis is disposed in valve-like folds, in such a manner as to pre- 

 vent the reflux of the bile, or of the contents of the intestine. 



655. The Liver may be regarded as essentially consisting of the ramifica- 

 tions of the Hepatic Duct, in close rela- 



[Fig. 137. tion with those of the Portal Vein and 



Hepatic Artery, that serve to convey 

 blood to the minutest parts of this organ, 

 and with those of the Hepatic Vein, 

 which return it to the heart, after it has 

 been subservient to the Nutrition of the 

 structure and to the -elaboration of the 

 Secretion. Besides these, the Liver con- 



1 Nucleated cells composing the parenchyma ^ Lymphatics and Nerveg the ]atter 

 of the gland ; 2, lobules of human liver with , . J a r , . 



ramifications of the hepatic vein.] are chiefly derived from the Sympathetic 



system, and are distributed on the walls 



of the vessels and ducts. These various portions of the structure are con- 

 nected together by a fibrous tissue, to which the name of Glisson's Capsule 

 has been given. For our present knowledge of their ultimate arrangement, 

 we are almost entirely indebted to Mr. Kiernan,^ whose account of them will 

 be here followed. 



a. When the Liver is closely examined with the naked eye, it is seen to be made up of 

 a great number of small granular bodies, about the size of a millet seed, of an irregular 

 form, and presenting a number of rounded projecting processes upon their surface. 

 These are commonly termed lobules, although by some Anatomists they are spoken of as 

 acini. When divided longitudinally, they have a somewhat foliated appearance, arising 

 from the distribution of the Hepatic Vein ; which, passing into the centre of each division, 

 is termed the m/ra-lobular vein. The exterior of each Lobule is covered by a process of 

 the capsule of Glisson; and its substance is composed of the minute ramifications of the 

 before-mentioned vessels, arranged in the manner presently to be described; the spaces 

 between which are filled up with a parenchyma, composed of nucleated cells, as shown 

 in Fig. 138, 1. The structure of each lobule, then, gives us the essential characters of the 

 whole gland. 



.*. The Lobules, when transversely divided, are usually found to present somewhat of 

 a pentagonal or a hexagonal shape; the angles being generally somewhat rounded, so as 

 to form a series of passages, or m/er-lobular spaces ; "in these He the branches of the Vena 

 Portland of the Hepatic Artery and Duct, from which are derived the plexuses that com- 

 pose the lobules. Each Lobule, when examined with the microscope, is found to be ap- 

 parently composed of numerous minute bodies, of a yellowish colour and of various 

 forms, connected together by vessels; to these the name of acini was given by Malpighi; 



* In the first Giraffe dissected by Mr. Owen, no gall-bladder was found; in the second 

 there were two. 



f In the Horse and Dog this coat is clearly muscular. 

 t Philosophical Transactions, 1833. 



