78 The Microscopie Structure NO ee 
diameter decreases with that of the vessels which they line; in the 
larger vessels it ranges from 3/55 t0 zs'so of an inch, in the smaller 
ones it is not more than sz5 of an inch. ‘The finer veins, near 
their transition into the capillaries, are only lined by nuclei. I 
have not been able to demonstrate satisfactorily any muscular fibres 
in the walls of the portal ves. The vasa vasorum, by which they 
are nourished, are derived from the plexus of small arteries in the 
capsule, previously described. They penetrate into the external 
coat of the vessel to terminate in a large-meshed network of large 
capillaries. ‘The venules arising from these proceed to join that 
plexus of small portal veins imbedded in the capsule. The limited 
supply of blood sent to the walls of the portal veins may be attri- 
buted to the absence of the muscular element. 
The walls of the hepatic arteries, after the removal of the cap- 
sule, consist of three distinct coats. The first, or external, is 
fibrous and more dense than that of the portal veins, especially 
where it rests upon the middle or muscular coat. This is of con- 
siderable thickness, and consists of a layer of longitudinal, and 
another of circular, smooth, muscular fibres. The internal is the 
serous coat; it is lined by the same epithelium as the portal veins. 
The vasa vasorum are very abundant in the external coat of the 
hepatic arteries. Derived from the small arterial plexus within 
the capsule, they form a set of anastomoses in the external coat 
near its outer surface ; but, if the outer portion of this coat is raised, 
a network with smaller meshes is found beneath. These meshes 
vary in size; neither are the vessels composing them uniform in 
diameter, some being 33>, others z;'oo of an inch thick. As the 
network approaches the muscular layer, its vessels become con- 
siderably smaller, their diameter ranging from zq55 to sono Of an 
inch. In the muscular coat I have been able to detect only a few 
loops of vessels and no regular network. The veins proceeding 
from the vessels just described, join the plexus of small portal veins 
in the capsule, from which the inter-lobular portal veins arise. 
The walls of the hepatic veins consist of the same elements as 
those of the artery, with the exception that the muscular layers are 
considerably thinner, and the vasa vasorum not so abundant, the 
capsule also adheres more firmly to the fibrous coat than in the case 
of the hepatic artery and portal vein. 
As the branches of the vessels become smaller and their walls 
thinner, the muscular element in those of the hepatic artery and 
veins is gradually lost. In the same manner, the capsule is disap- 
pearing by becoming ultimately blended with the walls of the finer 
branches. 
The investigation of the structure of the finer branches of the 
blood-vessels is very tedious; the best manner of pursuing it is as 
follows :—A piece of liver is put under water, and a fine probe— 
