544 ELEMENTARY PHYSIOLOGY less. 



The cells nearer the dermis are more immediately and 

 abundantly supplied with nourishment from the dermal 

 blood-vessels, and serve as the focus of growth and 

 multiplication for the whole epidermis ; they are in fact 

 the progenitors of the superficial cells which, as they are 

 thrust away by the intercalation of new cells between 

 the last formed and the progenitors, become meta- 

 morphosed in form and chemical character, and at last die 

 and are cast off. 



And it follows that the epidermis is to be regarded as a 

 compound organism made up of myriads of cells, each 

 of which follows its own laws of growth and multiplica- 

 tion, and is dependent upon nothing save the due supply 

 of nutriment fi'om the dermal vessels. The epidermis, 

 so far, stands in the .same relation to the dermis as does 

 the turf of a meadow to the subjacent soil. 



8. The Unit used in Histological Measurement.— 

 Structures w^hich are rendered clearly distinguishable 

 only by a magnifying power of 300 or 400 diameters must 

 needs be very small, and it is desirable that, before 

 going any further, the learner should try to form a definite 

 notion of their actual and relative dimensions by com- 

 parison with more familiar objects. A hair of the human 

 head of ordinary fineness has a diameter of about ^Joth 

 (say 0"003) of an inch, or 0"08 mm. (millimeter). The hairs 

 which constitute the fur of a rabbit, on the other hand, 

 are very much finer, and the thickest part of the shaft 

 usually does not exceed yg'tfoth of an inch, i.e. 0"001 inch, 

 or about 0"025 mm. ; while the fine point of such a hair 

 may be as little as o^^goth of an inch, about 0"001 mm., 

 or even less in diameter. 



In microscopic histological investigations the range of 

 the magnitudes with which we have to deal ordinarily lies 

 between 0"1 and O'OOl millimeter ; that is to say roughly 

 between one two hundred and fiftieth and one twenty-five 

 thousandth of an inch. It is therefore extremely con- 

 venient to adopt, as a unit of measurement, 001 



