102 COMPARATIVE PHYSIOLOGY 



invertebrates deals with the action of drugs and nervous 

 stimulation on the heart. Molluscs, Arthropods, and Annelids 

 are probably the only invertebrate groups in v^hich the cir- 

 culatory system plays an important role. 



Other things being equal, the rate at which a tissue can take 

 up oxygen depends upon the amount of blood which flows 

 through it in unit time. The flow of liquid through a tube 

 depends upon the force propelling it, the viscosity of the fluid, 

 the sectional area of the tube, and the length traversed. The 

 last can be regarded as a constant for present purposes. The 

 second is probably an important but little known factor in the 

 circulation. Attention has mainly been focussed on the propel- 

 ling force, supplied in vertebrates, arthropods, and mulluscs, 

 by the heart beat, and on the sectional area (constriction or dila- 

 tation) of the vessels. As the activity of the heart is an inter- 

 mittent force, the average force exerted by its action depends 

 upon two variables, the amplitude or force of the individual 

 beat, and the frequency with which the beats occur. The 

 output of the heart ir %he vertebrate depends partly upon the 

 resistance against which it works, being in so far a self- 

 regulatory mechanism, and is partly determined by a double 

 nervous control — that of the vagus, which is present in all 

 craniates including cyclostomes and is inhibitory, and that 

 of the sympathetic, which is an augmentor factor specially 

 well developed in the warm-blooded vertebrates. Variation 

 in the diameter of the vessels determines the resistance against 

 which the heart works as well as contributing directly to the 

 rate of flow through, and therefore the oxygen supply of any 

 given organ. It is provided for by the fact that the arteries, 

 veins, and capillaries possess contractile elements, plain 

 muscle in the case of arteries and veins and a special type 

 of contractile tissue, the cells of Rouget, in the case of the 

 capillaries. The relaxation and contraction of the contractile 

 elements investing the walls of the vessels is brought about 

 by extrinsic agencies (nerves and hormones) and by the local 

 action of metabolites produced during the activity of an 

 organ, as illustrated by the work of Bar croft upon salivary 

 secretion (see Chapter III). The influence of hormones is 



