ERECTILE TISSUE
Erectile or cavernous tissue is a vascular specialization in which many close-packed, endothelium-lined spaces are set in continuity with the bloodstream. The spaces are usually closed, but as they are directly fed by arterioles they rapidly engorge under appropriate nervous stimulation.
Erectile tissue is best known in connection with the genital system; it provides a large part of the structure of the penis (p. 193) and of the smaller but comparable female equivalent. In modified form it is also found in the teat wall, the nasal mucosa, the vomeronasal organ, and a few other sites. A simultaneous response of the genital and nasal erectile tissue is common and has provoked curious speculation; the association is less surprising than it may seem at first because the perception of odors plays a significant part in the sexual behavior of many animals.“Blood-cushions” formed by a concentration of veins, although not strictly comparable, may be mentioned here. Several of these arrangements are associated with the gastrointestinal tract. One of veterinary interest is provided by the ileal papilla of the horse (p. 556), which has a considerable capacity for engorgement. Another, less relevant example is supplied by the human anal mucosa; pads formed by the underlying veins are believed to contribute to closure of the orifice, and it has been claimed that the postnatal elaboration of these veins is correlated with the development of continence by the infant.