•895-]- 459 
ON THE INTROITUS VAGINAE OF CERTAIN 
MURIDAE.i 
BY GEREIT S. MILLER, JR. 
The vaginal orifice in the smaller mice and voles is tightly 
closed, except when mechanically torn open during copulation 
and parturition. Throughout pregnancy, lactation, and the period 
of sexual inactivity occurring in the late autumn and winter, the 
introitus vaginae is in the great majority of individuals so 
completely obliterated that the region between the anus and 
clitoris closely resembles the perinaeum of the male. The 
smaller size of the sexual eminence and the shorter distance 
between this and the anus are the only external characters by 
which the females are to be distinguished from the males at 
times of sexual inactivity when the testes are not in the scrotum. 
The membrane closing the vaginal orifice is, however, always 
easily ruptured, whereas considerable force is required to break 
through the skin of the male perinaeum. 
From December 20, 1890, to May 20, 1891, I examuied fifty- 
six female mice of the following species : Microtus pennsyl- 
iKinicus 4, JEvotorays gap'peri 11, Peromijscus leucopus 22, 
Peromyscus leucojyus canadensis 19. All but one of the female 
mice taken at EUzabethtown, N. Y., dui'ing December, Januaiy, 
and February, had the hymen- perfect, completely closing the 
orifice of the vagina. The one exception (Peromyscus leucopus, 
December 21) was an individual that contained two very large 
embryos, and may have just given birth to others. The hymen 
was completely destroyed, and blood flowed freely from the 
vagina. The rupture and bleeding were possibly due to the 
pressure of the trap in which the animal was caught. 
In nine Peromyscus leucopus and one Microtus pennsylvanicus 
taken at West Dedham, Mass., January 30-February 1, the 
hymen was without exception perfect. 
1 Contributions from the Zoological Laboratory of the Museum of Comparative 
Zoology at Harvard College, under the direction of E. L. Mark, No. XLV. 
2 This name may be used for the sake of convenience, though the structure is not 
homologous with the hymen. 
