Associated with the reproductive apparatus are various glands which (1) provide a fluid medium for the locomotion of the sperm cells; (2) facilitate copulation by reducing friction; (3) produce odors that are alluring to the opposite sex; and (4) furnish nutriment for the developing young. These glands may be grouped according to their place of origin into those (1) in the sperm duct or oviduct; (2) in the urogenital canal; or (3) in the integument.
Originating in the Sperm Duct or Oviduct
In many cases, for example in ruminants, most rodents, dogs, bears, martens, and shrews, the outer end of the sperm duct, near its entrance into the urethra, enlarges into an ampulla, which serves as a temporary reservoir for the sperm. This is lined with ampullar glands that secrete mucus. Such glands are absent in the cat, mole, European hedgehog Erinaceus, and the pig. In most mammals a saclike seminal vesicle (Fig. 404), lined with mucous glands, empties into each ductus deferens just beyond the ampulla. Between the seminal vesicle and the urethra the sperm duct is known as the ejaculatory duct. In general these vesicles have a glandular rather than a storage function, although sperm cells are frequently found therein. In the case of bats and some mice, the seminal vesicles enable these animals to exercise a sort of “birth control,” in that after copulation the mucus that they produce forms a gummy plug which fills the entrance to the uterus, the cervical orifice, and prevents for a considerable time subsequent impregnation. There are no seminal vesicles in monotremes, marsupials, cetaceans, or carnivores. In man they appear first about the end of the third month of fetal life.
In the human female there are present at least three sorts of glands, namely, uterine, cervical, and vestibular, associated with corresponding regions of the oviduct.
The uterine glands are tubular structures lining the uterus. They have to do with the epithelial regeneration of the uterine walls during menstruation, rather than with glandular secretion.
The much branched cervical glands, found in the cervix of the uterus, produce mucus which moistens the inner surface of the glandless vagina, while the scattered vestibular glands, located in the vestibule near the clitoris and around the outlet of the urethra, perform a similar function.
Originating from the Urogenital Canal
The glands of the urogenital canal are the prostate, in the male, and the urethral glands, which appear in both sexes.
The prostate gland, in man at least, is the most important of all the accessory reproductive glands. It is a compound tubulo-alveolar gland about the size of a horse-chestnut, made up of thirty to fifty lobules and opening into the urethra by means of two large and fifteen to thirty lesser ducts. It is embedded in a tough capsule of connective tissue and abundantly supplied with blood, nerves, lymph, and more smooth muscle cells than any other accessory reproductive gland. It surrounds the beginning of the urethra at the point where the ducti deferentes, or sperm ducts, enter.
The secretion produced by the prostate gland, forming a large part of the semen, or spermatic fluid, is a thin milky emulsion, faintly alkaline with a characteristic odor. The prostate gland is well developed in rodents, bats, perissodactyls, primates, and most carnivores. It is less well developed in ruminants, and is absent in monotremes, marsupials, edentates, and among carnivores, in the marten, otter, and badger. In man the prostate gland frequently becomes hypertrophied in old age and may deposit concretions of calcium phosphate, causing trouble by pressure upon the urethra which it envelops.
The urethral glands are of two sorts. First, mucous glands, called glands of Littre, in the male, that are most numerous in the dorsal region of the urethral wall along the penile part of its course; and second, paired bulbourethral glands, called Cowper’s glands in the male and Bartholini’s glands in the female.
Cowper’s glands are two small structures about the size of peas in the human male, with ducts an inch long opening into the urethra at the base of the penis. They produce a clear glairy mucus during sexual excitement which protects the sperm against traces of acid that may be present. Bears, dogs, and aquatic mammals lack Cowper’s glands, but in other mammals they are quite generally present, being especially large and active in rodents, elephants, pigs, camels, and horses. They are the only accessory reproductive glands in the male Echidna.
Corresponding to Cowper’s glands of the male, are the glands of Bartholini in the female, that open into the vestibule in the groove on either side between the hymen and the labia minora. They produce mucus which functions largely as a lubricant during copulation.
In the formation of the semen it has been shown that the first contributions come from Gowper’s glands and the glands of Littre, followed by the secretion of the prostate gland before the sperm from the ductus deferens are added, while the final glandular contribution is from the seminal vesicles.
Originating in the Integument
Various odoriferous glands of integumentary origin, named in different cases anal, inguinal, perinaeal, and cloacal glands, occur among vertebrates. These are usually located around the anus or genital aperture and serve to stimulate the opposite sex. The famous scent glands of skunks belong to this category, as do also the anal glands of dogs, which are well known to be of paramount importance in the social life of these animals.
Tyson's glands are small sebaceous glands that are situated around the base of the glans on the penis in the depths of the preputial fold of skin. They also produce an odorous substance called smegma.
The male alligator has submaxillary glands at the edge of the lower jaw on either side, which enlarge and emit a musky odor during sexual excitement.
Finally, under integumentary glands there should be mentioned the mammary glands, already described in a previous chapter, that have a place in the general scheme of reproduction in the mammals, since they provide sustenance for the newly born young. Marsupials, which have no true placenta, nourish the fetus before birth with “uterine milk” produced by uterine glands.