PLACENTA (NORMAL ANATOMY). 



715 



clitoris. The nymphae are subject to the 

 same inflammatory and specific disease as the 

 labia, but they more commonly affect the sur- 

 faces than the substance of these parts, which, 

 being of a denser texture than the labia, are 

 not so easily infiltered with the venous, san- 

 guineous, or purifonn fluids, that readily collect 

 within the latter. 



The morbid conditions of the vestibule con- 

 sist chiefly in inflammatory hypertrophy of 

 the vestibular follicles, especially of those 

 which immediately surround the urethral ori- 

 fice and line the sides of the ostium vagina?. 

 They present the appearance of small red gra- 

 nulations. A more decided spongy vascular 

 growth often springs from the border of the 

 urethral orifice, where it forms either a bright 

 red fringe, or a soft tumour, varying in size 

 from a pea to a cherry (vascular tumour of 

 the meatus). The vulvo-vaginal gland and 

 its duct may be the special seat of blennor- 

 rhoea, sometimes of an infecting kind. This 

 may be made to ooze from the orifice of the 

 duct, by pressure behind the labium. 



The most frequent varieties in the condi- 

 tions of the hymen and ostium vagina have 

 been already noticed.* The vaginal orifice 

 may be nearly or completely obstructed by an 

 adventitious membrane, or by the hymen pre- 

 ternaturally developed. Some of these states 

 are congenital, but others are acquired. In 

 either case, attention is often not called to 

 them until after the establishment of puberty. 

 According to their degree, they interfere with 

 the functions of the vagina, partly or alto- 

 gether preventing intromission, and rendering 

 insemination imperfect or impossible. They 

 impede the exit of the products of conception 

 and the escape of the menstrual fluid. In 

 the latter case, when menstruation is esta- 

 blished, the fluid collects in and dilates the 

 vagina and cervix uteri, and lastly the body 

 of the uterus, and even the Fallopian tubes. 

 Spontaneous rupture of the hymen, or mem- 

 brane, may then occur, liberating the fluid. 



PLACENTA. 



The placenta is the organ provided in 

 each pregnancy for the nutrition and respira- 

 tion of the foetus. To this it is connected on 

 its free side by the umbilical cord, while its 

 opposite or attached surface is united to the 

 fundus, sides, or lower part of the body of the 

 uterus by a layer of the decidua. A placenta 

 exists only in the mammalia and in some of 

 the cartilaginous fishes. It is composed of 

 structures derived partly from the ovum and 

 partly from the uterus. The fetal or embry- 

 onal portion is not always furnished by the 

 same portion of the ovum. It is sometimes 

 constructed from the yolk sac, as in certain 

 sharks, and the vessels which ramify in it are 

 then the branches of the omphalo-mesenteric 

 artery and vein. In other cases, as in the 

 mammalia, the chorion supplies the fetal 

 portion, which is here rendered vascular by 



* See VAGINA and HYMEN. 



the umbilical vessels derived from the allan- 

 tois. The maternal or uterine portion of the 

 placenta is furnished by the decidua or lining 

 membrane of the uterus. These two portions, 

 viz., the fetal and the maternal, originally dis- 

 tinct, and, even in their subsequent union, pre- 

 serving a certain independence, become more 

 or less closely connected together by interdigi- 

 tating the one with the other. Their union 

 may be one of mere contact, the fetal portion 

 forming numerous projecting vascular folds 

 which in the form of laminae or tufts, or single 

 villi, are received into corresponding depres- 

 sions or sulci, equally vascular, formed in the 

 lining membrane of the uterus. Or it may 

 consist in a more intimate conjunction of these 

 parts, such as takes place in man, where the 

 decidua or maternal portion forms a lamina 

 which is spread over and united to the groups 

 of villi that constitute the fetal portion. In 

 the former case at the time of parturition the 

 two portions are separated, the fetal processes 

 being simply drawn out of the recesses which 

 contained them without laceration of either of 

 the tissues. But in the latter, the one part 

 cannot be expelled without carrying a con- 

 siderable portion of the other with it. 



Form. The mammalian placenta exhibits 

 numerous varieties of form. In most Rumi- 

 nants it is composed of numerous detached 

 placentulae constituting groups or bosses of 

 vascular villi that project from the surface of 

 the chorion, and are received into correspond- 

 ing cotulas upon the inner surface of the uterus. 

 In the Carnivora the placenta encircles the 

 foetus in the form of a broad flat belt. In 

 Pachydermata, Cetacea, and many other fami- 

 lies, the villi are nearly evenly distributed over 

 the whole surface of the chorion, so that the 

 fetus is everywhere surrounded by placenta. 

 In some Rodentia and Quadrumana the pla- 

 centa is double. 



In man the placenta forms a single discoid 

 organ, which in its natural position is slightly 

 convex upon the outer, and concave upon the 

 inner superficies. Its outline is generally cir- 

 cular or oval; it is sometimes reniform, cordate, 

 or more or less triangular. It is rarely bilobed 

 or multilobed. 



Dimensions and weight. The size of the 

 placenta is exceedingly variable, bearing usually 

 a certain proportion to the bulk of the child. 

 A full-sized oval placenta measures 7 7^" in 

 its shorter, and 8 9" in its longer diameter, 

 and measures 23 24'' in circumference. The 

 thickness is generally greatest opposite to the 

 point of entrance of the funis, where the organ 

 measures commonly 1 \\"> but it becomes 

 gradually attenuated towards the margin, 

 which is slightly rounded, measuring here 

 only 2 4'" in thickness. The weight of the 

 placenta ranges from 15 30 oz. or more. 



Foetal surface. Upon the fetal surface of 

 the placenta are observed portions of the 

 amnion and chorion, together with the root of 

 the funis and the principal branches of the 

 umbilical arteries and vein. 



Amnion. The amnion (Jig. 484. am) y after 

 furnishing the outer covering of the funis, 



