The yellow coat is bounded mternally by a third tunic which is white, having pre cisely the milk-white colour, and very nearly the consistence, of articular cartilage. It is of variable thickness, but often i'" or more in diameter. It is very tough and cohe rent in texture, and is with difficulty split by needles, breaking into irregular fragments. These, examined by the microscope, are seen to be composed of tough fibres of con nective tissue, whose arrangement in wavy lines may be perceived through the mass, but which are so closely connected together by a semitransparent membranous medium as to be inseparable into distinct fibrillm, except at the margins of the fragments, where they are tolerably distinct ; where also the connecting medium may- be seen in the form of a struc tureless membrane. Minute granules are every where seen scattered throughout the mass, and adherent to the detached fibrillm. Treated by acetic acid, the fibres become transparent and pale, their outlines being hardly distin guishable. Oval nuclei, rather scanty, lie in the direction of the fibres. The whole sub stance has the appearance of a tissue which is in a low state of vitality.
It is probable that the presence of this coat within the follicle has been the cause of most of the differences of opinion which have ex isted regarding both the seat and the nature of the yellow portion of the follicle of preg nancy. It seems to have been assumed, with out further examination by tnany who have written upon this subject, that the coat last described is one of the coats originally com posing the Graafian follicle ; whereas it is formed by the metamorphosis of the blood-clot, already described as occupying the centre of the follicle before even the ovum escapes. I have seen very distinctly the fibrillation of this clot soon after the follicle has closed. It is then found to be gradually becoming pale, the red particles disappear by degrees, the clot adheres firmly to the inner surface of the ovisac, and the mass is converted into the low form of tissue just described, which may either take the condition of a membrane lining the cavity and leaving a central space filled by transparent fluid, or the whole may be con verted into a solid body. Either of these forms may be observed, and the knowledge that each may occur disposes of the specu lative question as to the time when the cavity of the follicle is obliterated.
On the other hand, the yellow coat which has been often described by authors as altogether a new formation, deposited either between or external to both of the follicular coats, can be most easily traced through all its phases, be ginning in the ascending vesicle, as the original ovisac ; its structure filled with nucleated cells, which gradually become charged with oil droplets until the whole tissue assumes the peculiar yellow which is so distinct about the time of bursting of the follicle. And this
colour it never loses until the time of its complete obliteration approaches; but through all the subsequent changes of the follicle the same anatomical structure and the same rela tive position of parts is preserved.
In the original preparation from whichfig. 387. was taken, nothing served to distinguish the several coats better than their colour. The outer coat or theca folliculi was red ; the second coat, or ovisac itself, chrome yellow ; the now internal and newly formed coat was milk-white.
It remains to describe the cavity in the interior of the follicle, which, though some times obliterated, is more frequently found still existing at the fourth month of utero-gesta tion. In the specimen represented in./7g. 387. the cavity measured 3"/ in diameter and con. tained a clear gelatinous fluid. In other cases a cavity at this time no longer exists, but the centre of the ovisac is occupied by a tough white substance, whose origin has just been explained.
It will not be requisite to follow out minutely the remaining changes which the Graafian fol licle undergoes. After the fourth or fifth month of pregnancy a certain diminution in size be gins to be perceptible. The walls of the cavity approach nearer to each other, and the white lining becomes thinner, and begins to be folded into plaits which, radiating outwardly, are seen intermingling with the yellow colour of the proper ovisac (fig. 389.). The outer boundary of the follicle also now presents an irregular and somewhat angular and occasionally an oval outline. These changes proceed with much variation in different subjects ; but usually at the time of delivery the ovisac, though still yellow, has lost much of its• brightness, and the cavity, if it had existed, is replaced by a solid white stellate cicatrix (fig. 389.) caused by the folding of the white lining membrane which bounded the ovisac on its inner surface. That the yellow coat is still vascular at this time is proved by the fact mentioned in the preceding page.
In proportion as the entire generative or gans subside into a quiescent state, so the re maining changes in the ovary take place more rapidly. The yellow colour of the ovisac passes into a paler hue, and at last into white. The radiating cicatrix may still be traced for some time longer, until, at the end of four or five months after delivery, every appearance of this structure has ceased to be discernible.