Upon the surface of the most salient por tion of the projecting follicle (fig. 380. a) the peritoneum, as already stated, tuay be wanting; the arnica albuginea also beneath has become greatly attenuated, and is sometimes found completely eroded, whilst internally the yellow coat of the follicle is also observed to be thinnest about this spot. Every preparation, therefore, is made for the laceration of the follicle at a given point, the seat of which can also be further determined by the observation that in this place the conjoined metnbranes, previously highly vascular, have become more transparent, whilst their vessels, having be come atrophied by compression, now carry little or no blood.
A very slight force is now sufficient to produce the rupture of the follicle in this precise spot, and such a force is supplied by the gradual accumulation of fluid, whether albuminous or sanguineous, or both, within the cavity.
It is believed by Coste that when the ovi sacs have reached this point, which is the full term of their growth, they may remain stationary until a state of excitentent arises, produced partly by the inaturity of the ovum, rind partly by the approach of the sexes, and that it is under the influence of such an ex citement that the rupture of the follicle most commonly takes place. What probability there is for such a supposition will be here after more fully considered. Whether in fluenced by any external stimulus, or whether occurring spontaneously, and from causes existing within the follicle, the increase of its fluid contents becomes at length so great that the cavity is distended beyond measure, and its walls can no longer resist the pressure, but give way at the thinnest and most pro jecting part. But it is probable that another power comes also into operation to aid this process. The wavy outline which has been already noticed Cflg. 381. c) as presented in a slight degree by the still unbroken ovisac, to gether with a certain amount of thickening of this coat, indicates a growth of this more rapid in proportion than that of the outer layer or tunic of the ovisac. This, therefore, will in some degree add to the pressure, be cause the outer layer of the follicle not being distensible beyond a certain limit, any in crease of the contents, whether fluid or solid, will alike contribute to augment the force which is brought to bear upon the weakest point of the walls.
As soon as the rupture has taken place, and the opening in the coats of the follicle and in the corresponding portion of the ova rian coverings is sufficiently large to admit of the passage of the ovum, the latter escapes, together with portions of the membrana gra nulosa.
On one occasion Pouchet was so fortunate as to meet with an opportunity of observing the ovum as it was in the act of escaping from the ovisac, and was lying between the margins of the lacerated opening.
Of the five coats which together compose the ovarian and follicular walls, four only, it will be observed, can offer any obstacle to the escape of the ovum; because the rnembranh. granulosa, which is the innermost of all, con tains rather than covers the ovum, whose escape cannot be impeded, but will be rather assisted by that membrane. Barry explains the mode in which this probably occurs as follows: —The ovum, imbedded in the cu.; mains and granular disc which form the centre of the membrana granulosa, at the moment when the laceration occurs, experiences the vis ci tergo occasioned by the pressure forward of the fluid, endeavouring to escape from within the follicle. This pressure is increased by the thickening of the inner wall of the follicle, amounting in some instances to an exuberant growth, which will act upon the ovum through the medium of this fluid. The obstacle to the escape of the ovum which had up to this moment existed, being rernoved by the laceration and absorption of the ovarian and follicular walls, that portion of the mem brana granulosa hich lies immediately behind the lacerated coats, where the ovum is im bedded, presents a surface for the operation of the vis tergo more or less considerable, according to the extent of the rupture.
And now the elasticity of the coats of the follicle, together with some pressure from the weight of the parts surrounding its base, come in aid of this force, and complete the expulsion of the ovum, which escapes together with a portion of the rnembrana granulosa, and passes into the infundibular end of the oviduct.
Fig. 383. shows the mode in which this pro cess occurs in the rabbit. Here is represented a portion of a ripe Graafian vesicle, which was upon the point of discharging an ovum. The follicle, after being dissected out of the ovary, has been subjected to slight lateral pressure in the compressorium, by which the follicle has been burst at the point (h) preparing for rup ture. The ovisac has given way at the thin nest point, and the ovum, surrounded by the tunica granulosa (g, 1.), and dragging after it portions of the retinacula (g, 2.) is shown in the act of escaping from the follicle.