22 Proceedings of Royal Society of Rdinhurgh, [sbss. 
in a sheath of lymph follicular tissue which is between the hilar sheath 
and the vessel. The arterioles are ensheathed doubly in a similar 
manner, but the adenoid sheath varies in thickness owing to the 
development of nodular swellings, — the Malpighian bodies or splenic 
follicles at irregular intervals. The follicles are enclosed in a 
contractile envelope chiefly composed of spindle-shaped muscular 
fibres derived from the hilar sheath. 
In all mammals, vutli possibly one exception — the guinea-pig, in 
some birds, and in some cartilaginous fishes, the lymph follicular 
sheath of the arterioles is developed into splenic follicles. But in 
osseous fishes, in the amphibia, in the chelonia, and in some birds, 
the lymph follicular sheath shows no special development. 
Lymphoid cells occur everywhere in the adenoid sheath, but most 
numerously in the follicles, which mainly consist of them. In the 
follicles the cells are divisible into two groups, — a central area of 
larger cells or leucoblasts forming the germinal centre of Flemming, 
and a peripheral area of small lymphoid cells resulting from division 
of those in the germinal centre. The lymphoid cells are extruded 
into the surrounding pulp partly by the centrifugal pressure of the 
formation of daughter cells, and partly by the compression of the 
follicles by their muscular covering. Multinucleated giant cells are 
sometimes found within the follicles, especially in the spleen of 
young mammals. 
The terminal portions of the splenic arterioles are invested by a 
continuous sheath, consisting of a homogeneous ground substance 
containing a few lymphoid cells, as in the osseous fishes, in the 
chelonia, and in some birds ; or by such a sheath developed into a 
nodose circumscribed swelling containing a few lymphoid cells and 
concentrically arranged spindle cells — probably muscular. These 
bodies were termed splenic “ ellipsoids ” by W. Muller, and “ capil- 
lary sheaths ” by Schweigger-Seidel. The axial vessel usually 
divides in the substance of the ellipsoid, and leaves it as thin-walled 
veins that open into the spaces of the pulp. It gives off capillary 
channels that anastomose with each other, and open into a blood 
sinus that surrounds the ellipsoid. This blood sinus communicates 
with adjacent ellipsoidal sinuses and splenic veins, but apparently 
not with the spaces in the pulp, nor with the emergent vessel of 
another ellipsoid. 
