MECHANICAL PROTECTION AND POISONS 



385 



and show no vacuoles, granules, or other marked signs of secretory 

 activity (Fig. 350, B). They secrete the poison, which fills the large cavi- 

 ties of the gland and is 

 sent through a duct to the 

 upper opening in the fang. 

 When the snake strikes, a 

 powerful muscle com- 

 presses the gland and 

 forces the venom out of 

 the fang in a jet. The 

 fangs are constantly being 

 formed as teeth are, and 

 several half-developed 

 ones are always to be 

 found in the gum, ready 



FIG. 350. A, outline of several lobules of the poison 

 gland of a rattlesnake, Crotalis horridus, as they appear 

 in a section of the gland; B, five cells from the epithe- 

 lium of these lobules, enlarged to show their finer struc- 

 ture. The colloid substance distad of the cells is the 

 poison. X 1500. 



to come out and take the 

 place of the old one when 

 it is lost. 



Another reptile, the 

 Gila monster, has poison 

 glands developed in the integument of the mouth near the lower teeth. 

 A bite from this creature is only venomous when it succeeds in turning 

 on its back and thus draining the poison into the wound. 



The birds and mammals produce no poisons but have many of the 

 mechanical integumentary forms of defense and offense. Among these 

 may be merely mentioned teeth, claws, spikes, spurs, spines, two dis- 

 tinct kinds of horns on top of the head, and one on the nose, to mention 

 but a few of them. We shall briefly describe but two of these structures, 

 the claw as exemplified by the human nail, and the spine of the porcupine. 



The nail is a cornified outer layer of stratified epithelial cells, developed 

 in a folded area of the skin on the ends of the fingers. Two regions of 

 this fold are distinguished; one, by the fact that the lower posterior 

 body of the nail is formed there and grows forward out of it, and the other 

 by the fact that the nail is not materially added to as it grows past this 

 region. Both these regions are on the under side of the fold, and are 

 known as the nail bed (Fig. 351). ,. 



The other side of the fold also rests against the nail, on its upper 

 surface, but this epithelium does not contribute in any way to the nail's 

 formation other than to protect it from drying during the weak begin- 

 nings of its growth. This surface is widest over the proximal "root" 

 of the nail, and forms overlapping ledges on each side. Its epidermal 

 layers are practically the same as on the rest of the skin. 



The basement membrane of the nail bed is thrown into longitudinal 



