INTEGRATION OF ACTIVITIES 



153 



PINEAL 

 PITUITARY 



THYROID 

 PARATHYROIDS 



■THYMUS 



been necessary in earlier chapters to point out some of the initiatory- 

 actions of this substance which may be here recalled. Increased con- 

 centration of carbon dioxide in the blood causes centers in the medulla 

 to increase breathing movements and to contract the blood vessels. 

 Here the effect is produced through the nervous system. Sometimes 

 carbon dioxide may act directly, without mediation of nerves, as when 

 it stimulates stronger heartbeat by direct action on the sinus node, 

 and almost directly when, perhaps by increasing acidity, it locally 

 causes dilation of blood vessels. There is thus an important chemical 

 regulation of muscle action, partly through, partly independent of, the 

 nervous system. Coagulation of the blood is also initiated by chemical 

 substances liberated from disintegrating platelets and injured tissue cells, 

 in conjunction with certain substances in the blood plasma. There are 

 some physical agents, also, which exercise regulatory control either 

 directly or through the nervous system. 

 Thus slightly higher temperature of the 

 blood, warming the thalamus of the fore- 

 brain, starts activity of the sweat glands, 

 which lowers the temperature; and 

 higher blood pressure in the great 

 arteries, acting through nerves, slows 

 down the heartbeat. And, finally, 

 greater warmth of the blood, influencing 

 the sinus node directly, not through 

 nerves, accelerates heart action. All 

 these influences have been discussed 

 before. 



Besides these chemical and physical 

 agents, which are all part and parcel of 

 the general physiological mechanism of 

 the higher animals and which mostly 

 serve other ends besides regulation, 

 there is a group of chemical substances 

 which have no other known function 

 than to exercise control over something. 

 These substances are know^n as hor- 

 mones. In general they are produced at 

 one place, but stimulate action at another, to which they have been 

 carried by the blood. One of the earliest of these substances to be 

 discovered was secretin, whose action in stimulating the pancreas and 

 liver has been described (page 104). 



While it is possible that most tissues produce substances that have 

 some influence elsewhere, the marked and well-known instances of 



•ADRENALS 



PANCREAS 



OVARIES 

 (in female) 



-TESTES 

 (in mole) 



Fig. 126. — Location of endocrine 

 glands in human body. Dotted lines 

 represent kidneys (above) and ovi- 

 ducts and uterus (below) to show 

 positions of glands. 



