THE CAPILLARY CIRCULATION 



By W. M. BAYLISS, F.R.S., M.A., D.Sc, 

 Professor of General Physiology, University College, London 



Although the part played by the small arteries, arterioles as 

 they are usually called, in regulating the height of the blood- 

 pressure and the supply of blood to the tissues of the various 

 organs is fairly well known, that of the capillaries and the 

 veins remains more obscure. The fact that the arterioles pos- 

 sess a muscular coat of relatively considerable thickness, while 

 the capillaries consist merely of a single layer of epithelial cells, 

 renders the mechanism of contraction and dilatation of the 

 former easy to realise. Their nervous supply has also been 

 worked out in some detail, and the facts of their active changes 

 in calibre under the stimulation of various nerves and under 

 the influence of different kinds of drugs and other chemical 

 substances are generally admitted. 



The veins also possess a muscular coat, but of much less 

 thickness than that of the arterioles. Some evidence exists 

 that they are capable of changes in calibre ; but the object of 

 the present article is to consider rather the facts that have 

 come to light in recent times with regard to the properties of 

 the capillaries, together with the consequences, theoretical 

 and practical, that follow from them. 



The difficulties in supposing that the capillary blood-vessels 

 have the power of altering their calibre independently of the 

 pressure of the blood entering them appear to be largely due 

 to the absence of a muscular coat. On the other hand, a 

 fairly copious supply of nerve fibres to them has been de- 

 scribed, and we know that cells other than muscle cells can 

 change their form when stimulated. The pigment cells of the 

 skin in fish and frogs may be mentioned, and the spherical 

 forms assumed by the amceba and the leucocytes are familiar. 

 Thus the absence of a muscular coat in the case of the capil- 

 laries does not warrant a denial of the possibility of active 



changes in calibre in the latter. 



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