DIAGNOSIS AND TREATMENT OF STERILITY IN STALLION AND BULL     187

matory conditions such as orchitis, epididymitis as well as infection elsewhere in
the body may inhibit or even abolish normal spermatogenesis.

                              HEREDITARY FERTILITY FACTOR.

That sterility may be due to structural malformations has already been men-
tioned. Gross imperfection of the genitalia, severe hernia and improper descent
of the testicles may be the expression of genetic action. According to Crew [1924]
grades of fertility are definite racial and breed characters and are transmitted in
inheritance and imperfections of the external genital organs are a part of the
general conditions of intersexuality and of hermaphroditism, both of which are
definitely genetical in nature. Richter [1926] and Wester [1921] lay great emphasis
on heredity in the causation of sterility of male goats and bulls. Crew points out
that chromosome aberrations lead to complete sterility and offers this as an expla-
nation of the common sterility seen in the offspring of an inter-specific cross, such
as a mule. He also brings forward cytological evidence in support of this con-
tention. In the case of another species cross, Bos americanus × Bos taurus, however,
there is probably a lethal factor as this cross is followed by the occurrence of
hydramnios during pregnancy resulting in maternal or foetal death.

                                 ENDOCRINE FACTOR.

The correlation of function between the reproductive organs and other glands
of internal secretion are well known. Among the endocrines that bear a special
relation to sexual development and function are the anterior lobe of the pituitary,
the thyroid, the prostate and the adrenal cortex. Disease of the pineal gland may
involve premature sexual development. The thymus gland has an inhibitory
action on the growth of the sexual organs and becomes markedly reduced at
puberty. There is little doubt that some cases of sterility are due to a derange-
ment of this correlation. Hormones have now been extracted from many tissues.
Evidence is available to show that the anterior lobe of the pituitary contains at
least three hormones—a growth-hormone, a gonad-stimulating hormone and a lacta-
tion hormone. Chemically they have not yet been separated. The testicular
hormones are complementary to the pituitary hormones and induce sexual excite-
ment and secondary sexual characteristics even in the abnormal absence of the
testes.

                                    ENVIRONMENT.

This is a minor factor associated with deficiencies in soil and fodder but seasonal
and climatic changes present certain stimuli to sexual instinct. Todd and Reynolds
[1931] consider that advancing the covering season for getting foals earlier in the
year, as has been done in England, has resulted in less fertility of stallions. It is

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