36 THIRD LECTURE. 



The matter is readily explained. We first recognize the 

 slower, less energetic direction of the vibration ; the other, 

 more rapid and powerful, we do not yet perceive. In which 

 direction, however, will the current be driven ? Manifestly, 

 in the latter. Engelmann explains the more sluggish move- 

 merit as a vital act of the protoplasma ; the more rapid, as the 

 effect of elasticity. We agree with him. 



Later, as death approaches, both movements become dis- 

 tinct. The current finally ceases, only a to and fro fluctuation 

 still remains. 



The ciliary movement has nothing to do with the blood 

 current, or with nerve life. It perishes rapidly in warm- 

 blooded animals, often very slowly in the lower cold-blooded 

 creatures. An elevation of the temperature to 44 and 45 C. 

 kills them, likewise increasing cold. Everything which exerts 

 a chemical influence likewise produces a destructive effect, occa- 

 sionally with a temporary increase of the vibration — water, 

 for example. It is an interesting observation, that dilute 

 solutions of potash and soda may temporarily excite the 

 paralyzed vibratory phenomenon to renewed energy (Virchow). 



The true epithelium — that is, so far as it originates in the 

 corneous and intestinal gland layers — is developed very early. 

 Even in the human embryo of five weeks, according to 

 Koelliker, the surface of the body is covered by a double 

 layer of cells, a deeper one consisting of smaller rounded 

 structures, and an upper one of larger, flatter, indented bodies. 

 The former represents the primary rudiments of the rete 

 Malpighii, the latter, the horny layer. 



Closely related to the epidermis, and arising from it in the 

 third month of fcetal life, are the human nails. These obtuse 

 quadrangular plates are arched outwards, and lie posteriorly, 

 with the so called nail roots, embraced within a deep furrow 

 of the skin. At the sides, the furrow becomes from behind 

 forwards more and more shallow. The anterior border of the 

 nail remains free. The portion of the cutis covered by the 

 nail bears the name of the nail-bed. The latter shows lon- 

 gitudinal rows of cutaneous papillae. 



