Conditions in Glandular Tissue

In discussing the conditions which give rise to motivational excitants, we have thus far dealt exclusively with those occurring in smooth muscle tissue. Another group is found in glandular tissue. The stimulating effect of increased activity of the adrenals upon the neuromuscular mechanism has already been described. The data of pathology provide additional evidence that a disturbance of the normal hormone balance by either hypoactivity or hyperactivity of one of the endocrine glands is accompanied by increased restlessness. Pituitary extract is often used in the labor of childbirth because it increases the activity of the abdominal muscles and the uterus. Insulin, the active hormone secreted by the pancreas glands, produces convulsions if liberated in excess of the blood condition it seeks to counteract.

It is probable that the majority of glandular conditions in the normally functioning individual do not constitute very effective motivational excitants; that is, each secretion is balanced and counteracted by some other secretion, so that chemical equilibrium is maintained without recourse to creating a bodily need which finds expression in overt activity. There is, however, one very notable exception--the internal secretions of the sex glands. While the much discussed "sex urge" can be traced directly back to these secretions, the exact manner in which they are related to the reproductive mechanism is not clear. It can be shown in lower animals, at least, that the motivational excitants give rise to excessive activity which tends to lead to copulation. The running of the female rat while in heat is a familiar example of the principle.

The relation of internal sex excitants to general activity is a little more difficult to demonstrate in male animals, though a good deal of their running is undoubtedly of such origin. Experiments on the removal and transplantation of the testicles and on the injection of testicular extracts have shown that both the general activity of seeking a mate and the specific pattern of copulative behavior have a hormonal basis. Castrated animals are notably lethargic; and removal of testes in the frog abolishes the sexual clasping reflex, normally lasting from 7 to 14 days.

The understanding of the psychophysiology of sex behavior is complicated at the human and lower primate levels by the apparent subordination of the internal motivational excitant to specific forms of external stimulation. Whereas in the male frog an essential internal excitant is derived from the pressure of fluids in the interstitial cells of the seminal vesicles, in man the essential excitant seems generally to be of external origin. Physiologists often claim that sexual excitation differs from most of the other basic urges in that its primary cause is not a general tissue need of non-nervous character. They maintain that in man, at least, sexual excitation is more or less independent of internal conditions, and that erotic desire often persists even after closely repeated gratifications (which should have early equilibrated any internal tissue condition). This, of course, is exaggeration; and the truth lies somewhere between the extremes. There is a well-defined sex mechanism which may be aroused mainly by external mechanical stimulation, but it is regularly responsive to a summation of external and internal excitations. We must conceive of this mechanism as having adjustor centers in both the spinal cord and the brain. These centers are capable of summating cumulative excitation from afferent nerve currents from various sources and of being quiescent until their c.e.s. has reached a critical value. Impulses of moderate intensity from these sources are not sufficient to arouse the adjustor centers; but when impulses from a single source exceed a certain intensity, they can summate with those present in moderate degree from other sources and produce response. A clear indication of how this works is seen in connection with the mechanisms of erection and ejaculation in the male. If the afferent impulses set up internally as a result of conditions of the sex glands are sufficiently intense, it needs only the slightest external stimulus (such as chance contact) to release the reflexes; this is the case in nocturnal emissions. On the other hand, very weak internal excitants can produce the ejaculatory response when accompanied by sufficiently intense external stimulation, as in masturbation. The point to be emphasized, however, is that some excitation of both types is probably necessary. The close similarity between this summational mechanism and that for respiration is very apparent.

This description of specific mechanisms should not obscure the fact that hormonal agencies are directly responsible for the general restlessness and activity which tend to bring about an external release stimulus for sex behavior. Besides playing upon the spinal center, internal sex excitants rise to higher levels and condition movements, first random and then deliberate, which finally evolve a method of raising the excitation of the primary spinal center to the critical point. There is considerable evidence that in man the pattern of external stimuli utilized to raise such excitability is almost wholly acquired. The wide prevalence of homosexuality and masturbation strongly suggests that the basis of heterosexual preference is social suggestion, plus an experienced appreciation of the suitability of the opposite sex as a means of reducing sexual tension. Lower animals, possessing less versatility than man, must continue to act randomly until some special external pattern, such as the type of bodily movement displayed by the other sex, becomes connected by summation with the internal excitants to produce the consummatory responses.