. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. A Jack of All Trades 203 not ordinarily extend into the intercellular spaces of the epidermis. Conse- quently stimuli which affect the body must reach the deeper-lying nerve endings of the corium through the protective barrier of the epidermis. 2. Microscopic The cells of the epidermis are arranged in stratified layers, like the leaves of a book, with the most important and indispensable layer next to the corium. From it the other, more superficial, layers
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. A Jack of All Trades 203 not ordinarily extend into the intercellular spaces of the epidermis. Conse- quently stimuli which affect the body must reach the deeper-lying nerve endings of the corium through the protective barrier of the epidermis. 2. Microscopic The cells of the epidermis are arranged in stratified layers, like the leaves of a book, with the most important and indispensable layer next to the corium. From it the other, more superficial, layers are derived, together with such accompanying modifications as hair,' feathers, and nails. This remarkable life-giving restorative layer of germinative cells is called the Malpighian layer (Fig. 138), in honor of Marcello Malpighi (1628-1694) who first pointed out its significance, thereby erecting to his name a memo- rial far more enduring than an isolated mausoleum or a marble shaft. Epidermis ' Derma Subcutaneous—Jtr^s Fat. Corneum Lucidum Granulosum Q Malpighian Fig. 138. Diagram of the skin, showing how the Malpighian layer gives rise to the superimposed layers of the epidermis. If drawn in proportion, the derma would be several times as thick as the epidermis. The cell progeny arising from the germinative Malpighian layer are gradually modified while they are being crowded toward the exposed sur- face of the skin. Their walls become thicker at the expense of the cytoplasm, while the breakdown of their nuclei is accompanied by a sequence of chem- ical changes in the cell substance. Finally, each cell flattens until eventually only a dead scalelike remnant remains, like the collapsed skin of a grape after the pulp has been squeezed out. The squamous husks of the outermost dead cells thus formed are constantly breaking free from the underlying layers, being shed with no interruption of function of the skin as a whole, while at the same time a continuous renewal from the Ma
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