144 SPECIAL HISTOLOGY. 



or less extent, the epidermis is, so to speak, continually occu- 

 pied in replacing what is lost, or in growing, and thus ex- 

 hibits its vegetative life in a more remarkable manner. Which- 

 ever 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 Avatery fluid 

 which subserves the cutaneous evaporation), maintaining their 

 vital activity, and at the most causing temporary deposits of 

 pigment 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 pro- 

 ceed continuously, may even be called growth. It is in this 

 process that the vegetative life of the epidermis-cells is most dis- 

 tinctly 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, sometimes 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 presented 

 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 probable 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 



