OCR Text |
Show 1876.] FEET OF CERTAIN MAMMALS. 531 pressure of the centre of the sole of the fore foot, while the soft sides and heel are in close contact with the surface on which the animal is walking ; at the same time the flexor brevis manus elevates the greater part of the anterior half of the sole by pressing the extremities and sides of the outer and inner toes downwards and also drawing them towards the centre line of the foot, thus producing a condition of the sole of the foot analogous to the sucking-cup of a cephalopod. In the hinder extremities the same conditions are fulfilled, though evidently much less effectively. The homologue of the palmaris, the plantaris, is also very large, and arises from the external condyle of the femur; and its tendon similarly passes into the sole of the foot to form the plantar fascia from which the flexor brevis digitorum arises as the flexor brevis manus in the fore foot. The softness and elasticity of the sole of the foot (described by Bruce and Schweinfurth) permit of nice adaptation to slightly uneven surfaces, and render the suctorial action, by which cohesion is maintained, possible. As it appeared evident to me that this soft condition of the sole must depend not only on large development of the subcutaneous areolar tissue, but also on a permanently moist state maintained by the secretions of numerous sudorific glands, I was not surprised to find, on examining several horizontal and vertical sections of the integument of the sole of the foot, that the sudorific glands were exceedingly numerous, at least fifteen times as numerous in a given space as in the sole of the human foot, amounting to 40,000 in the square inch*. With such an enormous number of sudorific glands the sole of the foot is doubtless kept constantly moist, and in the most favourable condition for adhering to smooth or slightly uneven surfaces, when it is converted into a kind of suctorial disk by the action of the muscles as described above. It might appear strange that, with such a soft sole to the foot, the animal could run with impunity over hard and occasionally angular surfaces ; but I find that the sole is everywhere protected by a deep layer of epithelium, in no place less than -^ of an inch in thickness. The importance of the great number of sudorific glands is here again apparent; for, with such a deep layer of epithelium, the sole of the foot would soon become quite horny from the effects of constant pressure, were it not not kept constantly moist by an abundant glandular secretion f. In the very remarkable species of Bat, Thyroptera tricolor, first described by Spix"f:, we find the only known instance (in Mammalia) * I have arrived at this calculation from observing that sometimes five, but more frequently four of the openings of the sudorific ducts were contained within a measured space equal to the square of T \^ of an inch. If, therefore, we assume four as the average number in this space, it follows that exactly 40,000 are contained within a square inch. t The significance of the retia mirabilia in the extremities of Hyrax, first described by Hyrtl, will be at once understood when we consider how necessary it is to maintaiu a constant vascular condition of the foot in order to keep up the abundant secretion poured forth by the numerous sudorific glands. I Simiar. et Vespert. Brasil. 1823, p. 61, pl. xxxvi. fig. ix. |