402 - Muliicellular Animals, Especially Man 



suddenly retrogresses — on the twenty-fifth to 

 twenty-seventh day of the menstrual cycle. 

 This sudden withdrawal of progesterone pre- 

 cipitates a breakdown of the uterine mucosa, 

 and leads to the menstrual flow. Moreover, 

 when the corpus luteum subsides, another 

 Graafian follicle — freed from the inhibiting 

 effects of progesterone — begins to ripen; and 

 this marks the beginning ol the next men- 

 strual cycle (Fig. 22-2). 



The human ovum, subsequent to the time 

 of ovulation, remains fertile for a very limited 

 time. Ordinarily the egg is capable of being 

 fertilized for only about 24 hours, although 

 some three days are usually spent by the 

 ovum in passing through the Fallopian tube, 

 en route to the uterine cavity. Likewise, 

 sperm remain viable in the female reproduc- 

 tive tract lor only about 2 days. In view of 

 these factors the period of maximum fertility 

 is closely associated with the time of ovula- 

 tion, which usually occurs from the twelfth 

 to the sixteenth day of the cycle. However, 

 extremely early and extremely late ovula- 

 tions are known to occur in some cases. 



Progesterone as a Factor in Pregnancy. 

 Without progesterone, the decidual reaction 

 (p. 388) cannot be elicited in the uterine mu- 

 cosa; consequently an implantation of the 

 embryo cannot occur. Normally, however, 

 the corpus luteum is well developed at the 

 time when the embryo descends into the 

 uterus, and at this time the decidual reaction 

 (p. 389) occurs very readily. Now, in fact, the 

 uterine wall is so receptive that it can be 

 induced to envelop almost any small object, 

 however inert and foreign it may be. 



If the corpus luteum is destroyed very 

 early in pregnancy, the uterine mucosa retro- 

 gresses and abortion follows. But in a normal 

 pregnancy the corpus luteum continues to 

 be active for about five months, and by then 

 the placenta is producing adequate quanti- 

 ties of progesterone and gonadotropic hor- 

 mone (p. 411). Accordingly, the uterus 

 continues to enlarge; the uterine contrac- 

 tions are held in check; the mucosa is main- 

 tained; and the mammary glands continue to 



develop. Then near the end of the preg- 

 nancy, the corpus luteum involutes, and the 

 placenta becomes less active. Deprived of pro- 

 gesterone, the uterine mucosa retrogresses 

 and loses its capacity to nurture the fetus. 

 Then the musculature of the womb becomes 

 active, ami the fetus is forcibly delivered 

 through the dilated vaginal canal. 



Hormonal Control of Lactation. All mam 

 mals suckle their young with milk from the 

 mammary glands. The breasts undergo an 

 initial devlopment at the time of puberty, 

 and further development occurs during preg- 

 nancy. However, it is only alter parturition 

 (delivery of the fetus) that the mammary 

 glands are stimulated to produce milk. 



At least three hormones, namely estrogen, 

 progesterone, and luteotrophin (lactogen), 

 must act successively in preparing the mam- 

 mary glands for secretion (p. 411). At pu- 

 berty, estrogen produces an initial growth ol 

 the glands, but this growth soon reaches a 

 maximum. During the menstrual cycle there 

 is a slight waxing and waning of the mam- 

 mary tissue, following the rise and fall ol 

 progesterone, but during pregnancy pro- 

 gesterone has time to produce a much larger 

 effect. However, the actual production of 

 milk can only be elicited after parturition, 

 when the pituitary gland begins to liberate 

 larger quantities of the lactogenic hormone, 

 luteotrophin (p. 411). 



The rudimentary mammary glands of a 

 male dog or other laboratory animal are 

 sometimes used to test the potency of the 

 lactation hormones. By suitably timed injec- 

 tions of estrogen, progesterone, and luteo- 

 trophin, the mammary glands of the male 

 can be developed quite completely, and milk 

 can be obtained. 



THE THYROID GLAND 



The human thyroid is a small bilobed 

 gland that lies in the neck, along the sides of 

 the trachea. The two lobes are connected by 

 the isthmus, a narrow strand of tissue that 

 passes across the front of the trachea, just 



