Sir

Contrary to Richard Wassersug's declaration in News and Views (Nature 388, 826–827; 1997), the penis of the nine-banded armadillo is not the first biological structure known to have an investment of orthogonal fibres instead of the helical arrangement classical in a hydrostat.

The sac of dura mater containing the human cerebrospinal fluid and spinal cord has an orthogonal array of elastin fibres that was demonstrated by electron microscopy eight years ago (B. R. Fink & S. Walker, Anesth. Analg. 69, 768-772; 1989).

In the spinal dura the orthogonal pattern apparently helps to accommodate the increase in longitudinal tension occasioned by flexion of the vertebral column. The orthogonal investment of the penis, however, is composed of collagen fibres which, as Wassersug pointed out, provide the penis with maximum flexural stiffness when erect.