14 r II YSIOLOGY AND TOXICOLOGY 



length divided, it will be observed that the calibre of the canal does not enlarge, 

 until it reaches the gland, and that the appearance of increased size here described, 

 is due to a distinct thickening of the walls of the tube. 



Fig. 8. 



a, Epithelial cells of main duct, and of the receptacle at the base of the gland ; b, pigment-cells of the duct. 



Pig. 9. 



Non-striated Muscular Fibre-cells op the Sphincter of the Duct. 



Upon careful examination of the nature of this tissue, I found it to be formed 

 by an increase in the amount of fibrous tissue, and by the addition of a layer of 



