HISTOLOGY 



FIG. 351. Longitudinal, ver- 

 tical section of the young 

 nail and nail bed of an in- 

 fant, n.r., nail root or lun- 

 ula; w.,nail; w.6.,nail bed. 

 (From a preparation by 

 DR. H. E. JORDAN.) 



furrows, and the layer homologous to the stratum 

 Malpighi is developed from the basal layer. It 

 fills the furrows and covers the ridges, thus 

 forming a nearly level surface. From this sur- 

 face these cells are added to the nail above, in 

 such a manner that they overlap each other like 

 roof tiles. They become hardened and trans- 

 parent. 



Claws are formed much as are nails, also 

 some forms of horn, as the cow's horn. The 

 horn of the rhinoceros is more to be compared 

 to a collection of jointly formed hairs of great 

 size and strength. The spurs on the wing and 

 shank of birds have a bony core as does the 

 cow's horn. 



Even hairs can be specialized into a set of 

 very formidable weapons. In the porcupine, 

 Erinaceus, they are immensely long and strong, 

 and have pointed and barbed ends which are 

 very injurious to their victims. On account of 

 its size this hair is developed by an exterior or 

 lateral hardening of the superficial layers of 

 stratified epithelium on a deep-set papilla (Fig. 

 352). In this respect they resemble a feather 

 somewhat, especially as the papillar epithelium 

 is thrown into longitudinal ridges to strengthen 

 the shaft and form the distal barbs. 



Technic. The cutting of sections of the 

 materials mentioned in this chapter is often 

 very difficult owing to the extreme hardness 

 and toughness of 

 some of them. 

 Certain small or 

 delicate hairs, 

 feathers, and nails 

 may be ignored 

 and sections cut 

 as usual after any 

 good fixation in 

 the usual fluids. 

 With the larger 



and harder kinds, FlG - 3S2- Transverse section of the 

 ,, , , developing spine of a porcupine, 



the best general Erinaceus. (After DAVIES.) 



