THE CIRCULATION OF BODY FLUIDS 103 



exemplified by the recent work of Krogh (1922), who has 

 shown in Amphibia that the pituitary is necessary for the main- 

 tenance of capillary tone, and that injection of pituitary extract 

 leads to capillary constriction. 



Nervous control of the peripheral circulation in vertebrates 

 is complex. The constriction of arterioles is brought about 

 by stimulation of sympathetic fibres which have their cell- 

 stations in the chain ganglia. Dilatation is probably produced 

 in the main by a peculiar mechanism which is nervous but 

 not reflex in the strict sense of the term. It is believed that 

 certain sensory fibres have branches with motor terminations 

 in the peripheral vessels, and may thus propagate local dis- 

 turbances such as arise through mechanical irritation (anti- 

 dromic action). 



The state of constriction of arteries and capillaries has 

 been studied by the three principal methods : manometer 

 measurement of the blood-pressure, volume-changes of in- 

 dividual organs, and rate of flow from vessels artificially per- 

 fused. The recognition through the work of Krogh that the 

 capillaries are active agents in determining the resistance to 

 the flow of blood in the peripheral circulation shows that none 

 of these methods is wholly satisfactory ; and our views on the 

 regulation of blood-flow may require considerable revision in 

 the near future. Stimulation of the central ends of most 

 sensory nerves produces reflex rise of blood-pressure in 

 Vertebrates, by exciting the nerve-endings of vaso-constrictor 

 paths which emerge from the vasomotor centre in the bulb. 

 In mammals there is a special afferent nerve from the heart 

 exercising an inhibitory influence on the vasomotor centre 

 (the depressor branch of the vagus). 



Our knowledge of the circulatory system of invertebrates 

 is very slight. Practically nothing is known of the peripheral 

 circulation, so that the following account must be confined 

 to the properties of the circulating fluid and the control of 

 cardiac rhythm. 



Blood of Invertebrates. — The occurrence of respiratory 

 pigments in the blood of invertebrates has already been treated 

 under that heading. Two further points are of special interest 



