36 GENERAL STRUCTURE OF THE BODY. 



In a general way the function of glands is chemical. 

 They filter out by osmosis, selecting the useful parts 

 for secretion and the useless for excretion. In the chem- 

 ical action that goes on considerable energy is given off, 

 as is shown by the amount of pressure in the glands and 

 by the fact that their temperature is higher than that 

 of the blood. They all work in a reflex manner, being 

 under the control of the central nervous system. Thus, 

 what is eaten affects the nerve terminals in the mouth, 

 the sensation passes to the nervous system, and an im- 

 pulse is carried by the motor nerves to the salivary 

 glands. 



Most of the glands have ducts to convey away their 

 secretion to other parts of the body or to send excretions 

 out of the body, but there are also ductless glands, which, 

 though they seem to have some important function in the 

 process of metabolism, are not well understood. Most of 

 them seem to manufacture some substance that is 

 absorbed by the tissues and that plays an important 

 part in the bodily metabolism, though nothing is se- 

 creted by them externally. They are said to have an 

 internal secretion, whereas the glands with ducts have 

 an external secretion. The liver has both forms of se- 

 cretion, the bile which is sent out and the glycogen that 

 is stored. The ductless glands are the thymus and thy- 

 roid glands, the suprarenal capsules, and the pituitary 

 body in the brain. 



Nervous Tissue. Presiding over all the organs, mus- 

 cles, and blood-vessels, as the source of all action and 

 all sensation, are the nerves. Nervous tissue is of two 

 kinds: 1. the gray or vesicular, which originates im- 

 pulses and receives impressions, and 2. the white or fi- 

 brous, which conveys impressions. The gray matter 

 consists of large granular cells of protoplasm containing 

 nuclei, which give off many branches or dendrites. 

 From the under surface there usually comes one main 

 branch, the axis-cylinder process. These processes 

 sometimes give off branches and sometimes not, but 



