No one can gainsay the ingenuity of some undergraduate students. When I wastraining as a vet, I remember medical students putting on a play and one of thecast was listed in the programme as ‘Scrotum – a wrinkled oldretainer’. The writers obviously recognized a feature of the scrotum, thesignificance of which seems to have escaped wider attention. That is, that thescrotum is indeed wrinkled.

It is popularly held that testicles descended into a scrotum because they neededto be cooled, but this belief needs much more careful examination. In the firstplace, the question as to why testicles descended outside the confines of theabdomen is not a legitimate one to ask, because it is shamefully Lamarkian(teleological) and not, therefore, in keeping with Darwinian principle. We canonly ask how, not why. Since in the majority of, though not in all, mammals, thetesticles are scrotal, the question that needs to be put is: what advantage didthe evolutionary descent of testicles confer on the animals in which ithappened?

In trying to answer this question, the architecture of the scrotum and itscontents might provide useful information. First, the scrotum is more than apiece of stretched skin. It is modified to become exceptionally thin and has asub-layer of elastic fibrous tissue which contains slivers of involuntary muscle(the tunica dartos containing dartos muscle). When the dartos muscle contracts,the scrotum wrinkles; when it relaxes, the scrotum is able to stretch so thewrinkles become ironed out, either partially or completely. These changes in theconfiguration of the scrotum are always accompanied by movements of thecontained testicles.