OF THE SKIN. 151 



upon the vital relations of its cells, or those which it has with the corium ; 

 it is essentially a stable tissue, which does not change in its elementary 

 parts, but, somewhat like a cartilage, has all its vital energies directed 

 to its unchanged self-maintenance as a whole (thickness of the whole 

 epidermis, proportion of the rete Malpighii to the horny layer), and in 

 its separate parts. However, since a throwing-oif of the external 

 layers, if not necessarily, yet accidentally, takes place almost con- 

 tinually over the whole body to a greater or less extent, the epidermis 

 is, so to speak, continually occupied in replacing what is lost, or in grow- 

 ing, and thus exhibits its vegetative life in a more remarkable manner. 

 Whichever takes place, it is the corium and its vessels from which the 

 fluids required by the epidermis are derived. In every locality, we may 

 suppose, that a certain determinate quantity of plasma, corresponding with 

 the anatomical and physiological relations of the vessels of the corium 

 and the thickness of the epidermis, permeates the latter, and, when it 

 is not growing, simply fills its cells and plates (independently of that more 

 watery fluid which subserves the cutaneous evaporation), maintaining 

 their vital activity, and at the most causing temporary deposits of pig- 

 ment in the rete Malpighii. If, on the other hand, its outer layers be 

 removed, a certain amount of plasma becomes free and disposable, and 

 then regeneration takes place, which, if it proceed continuously, may 

 even be called growth. It is in this process that the vegetative life of 

 the epidermis-cells is most distinctly evidenced, particularly in the rete 

 Malpighii, where it is unquestionably most intense, exhibiting itself 

 especially in the multiplication and growth of the cells, and in their 

 chemical changes. In the horny stratum the phenomena are less strik- 

 ing, though it must not be considered to be inactive even in the uppermost 

 layer ; being by no means dead matter, as we evidently see, when under 

 certain conditions, especially under abnormal states of the corium ' 

 the source of its nutrition it sometimes becomes hypertrophied, some- 

 times completely dies away. We have not as yet, however, attained to 

 an exact insight into the vital manifestations of the epidermic cells, and 

 are therefore not in a condition to decide which of the phenomena pre- 

 sented by them are to be ascribed to their own activity, and which to 

 the nature of the plasma which nourishes them. The latter is certainly 

 of the greatest importance for the epidermis, and it is more than pro- 

 bable that most of its peculiarities, as, for instance, its typically different 

 thickness in different parts of the body, the different relations of the 

 stratum Malpighii to the horny layer, and its pathological states, depend 

 upon qualitative and quantitative differences in the plasma. Upon what 

 condition, furthermore, it depends, that in the Malpighian layer, the 

 changes of the cells are far more considerable than in the horny layer, 

 whose elements all closely resemble one another, is as little obvious as 



