ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 327 



outer and inner coat in which the course of the fibres is predominantly longitudinal. 

 The central coat has a specific staining reaction, and in general appearance differs from 

 the other two coats, its nuclei being far more numerous and up to 30 m long, and its 

 fibres more accurately parallel. It does not appear in all cases to run continuously 

 round the gall bladder, but is interrupted at intervals. It is the only one of the three 

 coats which is distinctly muscular, the other two being composed of connective-tissue 

 fibres. The inner coat is the most vascular. The connective-tissue fibres have shorter 

 and more irregular nuclei, and have a coarse, granular, wavy appearance. In some 

 methyl-blue-eosin preparations the connective-tissue nuclei stain blue, and the nuclei of 

 the muscular layer red. In other cases the order of the layers appears to be reversed 

 — the outer layers being muscular and the central one connective-tissue, and the 

 conditions may even vary in different regions of the same gall bladder. 



A number of measurements of the thickness of the various coats of the gall bladder 

 results in the following average : — epithelium, 26 /* ; inner fibrous coat, 29 m ; middle 

 coat, 16 m; outer coat (very variable), 41 fx; total thickness of the gall bladder, 

 112 Ai. These measurements apply to the right lateral free surface of the gall bladder. 

 On the left the outer coat passes externally into a very loose connective-tissue layer 

 transmitting the larger blood-vessels. 



Maas has a few notes on the histology of the gall bladder in his work on the 

 "pancreas" of Myxine (p. 6). He finds in "old specimens and sections in a favour- 

 able plane " two layers of unstriped muscle — one circular and one longitudinal. The 

 layer he misses is evidently the outer one, which I also have found sometimes 

 represented by a wide zone of very loose connective tissue. His description of the 

 other two as both muscular is doubtless well within the variability of the animal. 



5. Histology of the Hepatic Ducts. 



The hepatic ducts consist of a central single layer of epithelium, without a basement 

 membrane, almost directly under which is a rich vascular network. The epithelium 

 is surrounded by a coat of coarse connective-tissue fibres having an average thickness 

 of 60 /«, and which lodges the nerve bundles. This itself is enclosed by a very loose 

 connective-tissue sheath. When the hepatic duct is cut across a triple lumen is 

 disclosed. The largest is that of the duct itself ; the next is one of the two branches 

 of the common portal vein ; and the smallest is a branch of the cceliac artery. In the 

 neighbourhood of the artery and vein is found also an accompanying nerve. Further 

 smaller vessels and nerves are present, as above mentioned. 



The hepatic duct is circular in transverse section and measures about 300 /j. in 

 diameter in an average specimen. Its lumen is not regular, but is indented here and 

 there. The lining epithelium consists of tall narrow cells about 72 m high and 4 m 

 wide. The nuclei are at the base, and are in marked contrast with those of the gall 

 bladder. They are oval and vesicular, with a well-marked nuclear membrane and 

 reticular chromatin. The cytoplasm is granular and may be vacuolated, the cell 



