1876.] FEET OF CERTAIN MAMMALS. .531 



pressure of the centre of the sole of the fore foot, while the soft elastic 

 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 extre- 

 mities 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 con- 

 dyle of the femur ; and its tendon similarly passes into the sole of 

 the foot to form the plantar fascia from which the flexor brevis dlgi- 

 torum arises as the flexor brevis manus in the fore foot. 



The softness and elasticity of the sole of the foot (described bv 

 Bruce and Schweinfurth) permit of nice adaptation to slightly uneven 

 surfaces, and render the suctorial action, by which cohesion is main- 

 tained, possible. As it appeared evident to me that this soft con- 

 dition 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 an- 

 gular 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 thick- 

 ness. 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 abun- 

 dant glandular secretion f. 



In the very remarkable species of Bat, Thyroptera tricolor, first 

 described by Spix J, 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 -r ( Vrr 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 Hyra.v, first de- 

 scribed 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 Siminr. ef Vespert. Brasil. 1823, p. til, pi. xxxvi. fig. is. 



