CHANGES IN NON-PREGNANT UTERUS 103 



marked correlation between the cyclical uterine changes and the 

 developmental cycle in the corpora lutea of the ovaries (see p. 38). 

 During the second week after a heat period the cells of the 

 uterine and also the vaginal mucous membrane begin to show 

 signs of degeneration and the process of desquamation commences. 

 At the completion of two weeks the mucosa undergoes " wholesale 

 destruction," but it is not clear whether this is to be regarded as 

 prooestrous or pseudo-pregnant degeneration, or whether it is a case 

 of the two processes being compressed into one such as may possibly 

 occur in man. 1 



Cyclical changes in the sexual organs of the rat have also 

 been described by Long and Evans, 2 and by Kirkham and Burr. 3 



THE CYCLE IN UNGULATES 



The uterine changes have been worked out in the sheep and in 

 the sow. As described in the former 4 they relate chiefly to the 

 blood-vessels, and are grouped according to four periods as in the 

 case of the monkey, the dog, and the ferret, referred to above. 



(1) Period of Rest. The histological characters of the uterus 

 during this period are those of an organ in a state of quiescence. 

 Protoplasmic processes can be seen passing from certain of the stroma 

 nuclei, but these, though denser in some places than in others, show 

 little evidence of division. Dark brown or black pigment may be 

 present in considerable quantities, especially in the region subjacent 

 to the epithelium, both in the cotyledonary papilla; and (more 

 frequently) between them and round their bases. Such pigment has 

 not been observed in yearling sheep (i.e. in sheep less than a year 

 old) ; neither does it appear to occur, as a rule, during the anoestrum, 

 but only during the dioastrous interval. 



(2) Period of Growth. The nuclei in the stroma multiply, and 

 the mucosa increases slightly in thickness. The epithelium, however, 

 appears to remain unaffected. The blood-vessels increase both in 



1 Cyclical changes in the guinea-pig, more particularly in the ovary, were 

 previously described by Leo Loeb (" The Cyclic Changes in the Ovary of the 

 Guinea-Pig," Jour, of Aforph., vol. xxii., 1911). See also Ishii ("Observations 

 on the Sexual Cycle of the Guinea-Pig," Biol. Bull., vol. xxxviii., 1920). 



2 Long and Evans, "The (Estrous Cycle in the Rat," Anat. Record, vol. xviii., 

 1920. 



3 Kirkham and Burr, "The Breeding Habits, Maturation of Eggs, and 

 Ovulation of the Albino Rat," Amer. Jour, of Anat., vol. xv., 1913. For other 

 work on the oestrous cycle and sexual periodicity, etc., in Rodents, see abstracts 

 of papers by Long and Evans as well as of papers by Freyer, Sutter, and others 

 in Proc. Amer. Assoc. Anat., 1920 and 1921, Anat. Record, vols. xviii. and xxi. 

 Long and Evans have described pseudo-pregnancy in the rat as a result of 

 sterile coition and of mechanical stimulation of the cervical canal. 



4 Marshall, " The CEstrous Cycle and the Formation of the Corpus Luteum 

 in the Sheep," Phil. Trans., B., vol. cxcvi., 1903. 



