EXCRETION AND THE EXCRETORY ORGANS 



479 



is highly vascular and raised up into papillae which, except in the 

 region of the lunular, are arranged in longitudinal rows, slightly 

 diverging as they run towards the tip of the finger or toe. It is by 

 new cells formed on its bed and added to its under surface that the 

 nail grows in thickness, as it is pushed forward by the new growth 

 in length at its root. The free end of a nail is therefore its thickest 

 part. If a nail is " cast " in consequence of an injury, or torn off, a 

 new one is produced, provided the matrix is left. 



The Glands of the Skin are of two kinds, the sudoriparous or 

 sweat-glands, and the sebaceous or oil-glands. The former belong to 

 the tubular, the latter to the racemose type. The sweat-glands, 

 Fig. 144, lie in the subcutaneous tissue, where 

 they form little globular masses composed of a 

 coiled tube. From the coil a duct (sometimes 

 double) leads to the surface, being usually spi- 

 rally twisted as it passes through the epidermis. 

 The secreting part of the gland consists of a 

 connective-tissue tube, continuous along the 

 duct with the dermis ; within this is a basement 

 membrane; and the final secretory lining con- 

 sists of several layers of gland-cells. A close 

 capillary network intertwines with the coils of 

 the gland. Sweat-glands are found on all regions 

 of the skin, but more closely set in some places, 

 as the palms of the hands and on the brow, than 

 elsewhere : there are altogether about two and a 

 half millions of them opening on the surface of 



the Body. 



rm T T , , , . , 



The sebaceous glands nearly always open into 

 hair-follicles, and are found wherever there are 

 hairs. Each consists of a duct opening near below the dermis. 

 the mouth of a hair-follicle and branching at its other end : the 

 final branches lead into globular secreting saccules, which, like 

 the ducts, are lined with epithelium. In the saccules the substance 

 of the cells becomes charged with oil-drops, the protoplasm disap- 

 pearing; and finally the whole cell falls to pieces, its detritus con- 

 stituting the secretion. New cells are, meanwhile, formed to take 

 the place of those destroyed. Usually two glands are connected 

 with each hair-follicle, but there may be three or only one. A pair 



F IG . i44._A sweat- 



ghian layer; b, der- 

 mis. The coils of the 



land proper, em- 



