Finally, In The Domain Of Physiology, we encounter, upon the vascular plexuses in the lateral ventricles of the brain, a formation which, as comparative microscopical investigations teach us, fully coincides with the excrescences still before us. Upon the lateral plexuses of vessels are seated great multitudes of delicate, red, vascularized villi. These consist, beneath an epithelial covering, of a transparent, multifariously projecting, fretted, hollow growth, in the protuberances of which run arched bloodvessels of no inconsiderable size. Here we seldom fail, especially upon subjects advanced in years, to find numerous little cystlets, for the most part lodged in those protuberances. Generally speaking they do not outgrow a certain measure, a diameter of from 1 /25th of a millimetre up to a cyst discernible by the naked eye, and differing from what are usually called cysts of the vascular plexus. By dint of a repeated development of central nuclei, the majority are wrought into concentrically laminated growths, and undergo incrustation.

The very common so-called cysts of the vascular plexus are not genuine cysts. Numerous examinations have convinced us of their being dilatations of the hollow growth which constitutes the villus of the bloodvessel plexus. This dilatation pre-eminently affects the villi adjacent to the tortuous bloodvessels upon the convexity of the arch described by the vascular plexus. Accordingly, they are clusters of gibbous and indented vesicles, separated by the indentations into several loculi. Little remnants of villi are often to be detected upon them. We have frequently observed the dilatation of the villus at its commencement. In this manner, the so-called cysts of the vascular plexus answer to the pouches into which the excrescences constituting villous cancer widen. Besides other matters, they contain areolar tissue, sometimes to perfect repletion, so that, in this respect, they may be likened to the excrescences upon the internal membrane of the cysts, and especially to those which, through the development of areolar tissue, have become internally parenchymatous; as, for instance, in the cyst of sarcoma.

The contents of what are called cysts of the vascular plexus answer, in all essential points, to the contents of cysts proper. They consist in a watery albuminous fluid, which, with a certain amount and quality of its accompanying organic elements, becomes turbid, thickish whey-like, and in great measure supplanted by the development of areolar tissue.

These elements are -

(a.) Minute, 1 /800 millimetre-sized, elementary granules, free, or numerously held together by a viscid interstitial substance.

(b.) Larger, up to nucleus-sized, spheroid, occasionally somewhat oblong vesicles, which speedily increase to 1/25th of a millimetre. There are, besides, ordinary, granular, spherical, and oblong nuclei and granules.

(C.) The larger vesicles are simple, or successively enshrined. Some exhibit a double outline, that of the inner vesicle often lying so close to the outer one as to be easily overlooked. In other cases, the two are widely parted, the inner one being slightly wrinkled or curled. In others, again, there is, between the two contours, at some one spot, a space defined by a single outline, which sometimes resembles a shallow section of a sphere, and is particularly marked where the compressed contents of the external cyst are granular, and therefore contrast with the limpid contents of the inner vesicle. In some instances the inner vesicle is small, and either central to the outer one, or resting upon its wall. The latter kind probably engenders the form just adverted to.