400 PHYSIOLOGY. 



influence of the blood-pressure, and varies in its intensity as the 

 arterial tension varies. It, properly, speaking, is an accessory act of 

 secretion. 



2. The second feature is the activity of the gland-cells, which 

 take from the lymph the materials necessary for secretion, to change 

 them more or less. This phase is the essential act of secretion. It 

 is dependent upon nitration to the extent that filtration furnishes 

 the liquid which the glandular cells need and renews it when 

 exhausted. 



The activity of the gland-cells attains its maximum in general 

 during the apparent repose of the gland. When the gland is not 

 secreting, its cells are preparing substances peculiar to each secre- 

 tion. This is true particularly of the ferments, as pepsinogen, 

 trypsinogen, etc. 



The two processes filtration and gland-cell activity may be 

 separated from one another without producing any interference. 

 Thus, secretion can continue when the head is amputated, and even 

 if the circulation of the gland be arrested. Salivation can continue 

 after both these events have occurred. 



On the other hand, the injection of carbonate of soda into the 

 salivary duct destroys the gland activity without affecting the cir- 

 culation of the gland. Should the chorda tympani be stimulated 

 filtration from the blood continues, but the gland does not secrete. 

 There is an accumulation of lymph in the lymph-spaces until the 

 gland becomes cedematous. 



NATURE OF INTERNAL SECRETION. 



This is not the same for all of the glands. The secreted pro- 

 duct may be destined to destroy the noxious principles resulting 

 from the functions of the organ, as of the liver and suprarenal cap- 

 sules. Its aim may be to break up the excess of sugar, as is the 

 case with the pancreas; or to prevent excess of a colloid material, 

 as with the thyroid gland. The enrichment of the blood with use- 

 ful principles is accomplished by the sugar of the liver. The testicle 

 extract supplies more nervous energy. 



THE THYROID. 



The thyroid gland, when fully developed, has no excretory duct; 

 so, with the spleen, suprarenal bodies, and thymus, it is usually 

 classed under the head of ductless glands. 



The thyroid is a soft, reddish body embracing, the front and 



