272 DR A. J. WHITING ON THE 



times contain red blood-corpuscles, suggesting that the apertures in the wall of the 

 axial vessel may lead directly into the spaces of the network. There are, in addition 

 to the network, a few spindle cells arranged concentrically around the axial vessel. 

 There is a well marked limiting membrane, which is sometimes seen to be composed of 

 unstriped muscle. In most cases the ellipsoid is seen to be surrounded by a venous sinus. 



In the spleen of the Pig the ellipsoids resemble closely those in the sheep, but the 

 fibrous reticulum and the limiting layer are not so strongly developed. In their substance 

 are concentrically arranged spindle cells, lymphoid cells, and protoplasmic corpuscles. 

 There is an appearance of circular muscular fibres in the wall of the axial vessel, more 

 delicate and wider apart than those in the arterioles. A wide venous sinus surrounds 

 the ellipsoid and communicates with the veins of the pulp. 



In the spleen of the Dog the ellipsoids are round or oval in transverse section ; but 

 in longitudinal section their outline is irregular, being prolonged into angles at the places 

 of emergence of the vessels, so that their shape is sometimes like that of a multipolar 

 nerve cell. (Plate II. fig. 6.) A somewhat large arteriole enters the ellipsoid, and 

 divides into two or three branches which leave it at different points. As in the ellipsoids 

 of the pig, the axial vessel appears to possess a thin circular muscular coat. From the 

 sides of the axial vessels spring numerous delicate, wavy, capillary vessels, destitute of 

 endothelium, which anastomose to form a plexus and open into the peripheral blood- 

 sinus. In transverse section the lumen of the capillaries sometimes seems to be of 

 considerable size, quite large enough to transmit a red blood-corpuscle. 



The substance of the ellipsoid consists of a fibrous reticulum with connective tissue 

 corpuscles at its nodes, and lymphoid cells in its meshes, together with a small amount 

 of granular matrix. The reticulum is like that in the ellipsoid of the sheep, but is not 

 so strong and has smaller meshes. Appearing to form a part of the reticulum are a few 

 spindle cells arranged concentrically around the axial vessel. There is a limiting layer 

 formed of a somewhat strong membrane and spindle-shaped cells, or nuclei. Surround- 

 ing each ellipsoid is a capacious venous sinus, that contains cells similar to the cellular 

 elements of the pulp and red blood-corpuscles. The peripheral sinuses communicate 

 with each other by thin- walled channels, and similarly with the venous sinuses of the 

 pulp, and with the small veins that are laterally attached to the trabecule. One cannot 

 help being struck with the number of points of resemblance between the ellipsoids and 

 the follicles — the axial vessel, the radiating capillaries that anastomose, the peripheral 

 layer sometimes muscular, the fibrous reticulum, the lymphoid cells — but in each of these 

 particulars there is difference as well as resemblance. 



In the ellipsoids of the spleen of the Cat the arrangement of vessels is similar to 

 that in the dog, as also is their substance, except that it is a little denser. There is 

 a distinct limiting layer of spindle-shaped cells, but the peripheral blood sinus is not 

 always distinct in the adult cat (although it nearly always is in the kitten) ; it is 

 indeed sometimes nearly obliterated in the adult, apparently from compression by the 

 surrounding pulp. 



