THE HORMONES IN HUMAN REPRODUCTION 



section) from one side, stain it with appropriate dyes and 

 photograph it through the microscope (Plates XVII, XXI). 

 When studying the two-horned uteri of small animals such 

 as the rat, rabbit, or guinea pig, we usually cut our blocks 

 from the whole thickness of the tubular horns, as one slices 

 a banana, and therefore the thin sections for microscopic 

 study are round, with the uterine cavity showing in the 

 center (see Plate XVII, B, in comparison with A of the 

 same plate). 



We find that the surface is paved with a single layer of 

 tall cells, and that at frequent intervals this surface layer 

 pushes down into the depth of the endometrium, forming 

 fingerlike tubes, closed at the end, which reach almost to the 

 muscle (Fig. 12, C, D). These tubes are supported by spongy 

 connective tissue, and between them there is a network of 

 capillary blood vessels supplied by arteries. They are in fact 

 actually glands, able as shown in Fig. 13, to take water and 

 the "makings" of nutritive substances from the blood vessels, 

 build them up into foodstuffs for the early embryo, and dis- 

 charge the resultant secretion into the cavity of the uterus. 

 The endometrium is therefore something like a quick-lunch 

 counter, with a supply of raw foods in the rear (in the blood 

 stream), a row of cooks and waiters (the gland cells) and 

 a line of customers (the cells of the embryo). The outfit does 

 not however function in this way all the time; it secretes 

 nutritive materials, practically speaking, only when an egg 

 is likely to be present. How all this is regulated by the 

 hormones of the ovary will be explained in Chapter V. To 

 paraphrase a saying of Robert Boyle, the endometrium 

 looks like so much velvet, yet there are strange things per- 

 formed in it. 



The cervix and vagina. The lower end of the uterus pro- 

 jects downward into the vagina as shown in Fig. 11. This 

 part of the organ is known as the neck or cervix. Its lining 

 is full of glands which secrete mucus. 



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