342 



RESPIRATION AND EXCRETION 



is passed through the ureter to the urinary bladder; from this reservoir 

 it is passed out of the body, through a tube called the urethra. After 

 the blood has passed through the glomeruli of the kidneys it is purer 

 than in any other place in the body^ because, before coming there, it 

 lost a large part of its burden of carbon dioxide in the lungs. After 

 leaving the kidney it has lost much of its nitrogenous waste. So de- 

 pendent is the body upon the excretion of its poisonous material that, 

 in cases where the kidneys do not do their work p'foperly, death may 

 ensue within a few hours. 



Structure and Use of Sweat Glands. If you examine the 

 palm of your hand with a lens, you will notice the surface is thrown 



Sebaceous Gland 



Horny layer 



Pigment layer 



Tactile Organs'* 



Nerve 

 Blood Vessels 



Sweat Gtond-^1"' 



Fat 



Epidermis 



Dermis 



Subcutaneous layer of 

 connective tissue and fat 



Diagram of a section of the skin. (Highly magnified.) 



into little ridges. In these ridges may be found a large number of 

 very tiny pits; these are the pores or openings of the sweat- 

 secreting glands. From each opening a little tube penetrates deep 

 within the epidermis; there, coiling around upon itself several 

 times, it forms the sweat gland. Close around this coiled tube are 

 found many capillaries. From the blood in these capillaries, cells 

 lining the wall of the gland take water, and with it a little carbon 

 dioxide, urea, and some salts (common salt among others). This 

 forms the excretion known as sweat. The combined secretions 

 from these glands amount normally to a little over a pint during 



