August 25, 1923] 



NATURE 



271 



of the vessels forming this network were simply func- 

 tions of the general blood pressure driving blood 

 through the arteriole and of the state of contraction 

 of the arteriole itself. 



In reading the views on the circulation which 

 were general before the discoveries of Harvey, we 

 are often filled with astonishment that men endowed 

 with mighty intellects, like Leonardo da Vinci, could 

 not see what seems to us so self-evident. It is 

 difficult to comprehend how any one could dissect 

 the heart and be familiar with the effects of wounds 

 of different parts of the body and fail to perceive 

 the meaning of the valves in the heart and the 

 course of blood through this organ. Yet we our- 

 selves every day are equally blind. It is self-evident 

 that the colour, say, of the skin, depends not on the 

 amount of blood in the small arterioles but on the 

 fulness of the capillaries. Every one knows that the 

 capillaries may be overfilled together with constricted 

 arterioles, giving rise to blue cold skin, or that the 

 capillaries may be less full but with a vigorous circula- 

 tion through dilated arterioles, so that the skin is 

 warm and of the normal colour. These two observa- 

 tions should be sufficient to show that the state of 

 dilatation of the capillaries is not dependent only on 

 the condition of the arteries. Even a lifetime devoted 

 to science and research seems incapable of prevent- 

 ing us from accepting familiar appearances without 

 trying to understand them. It is not until some one 

 puts a definite question and our curiosity is aroused 

 that we become aware of a problem to be solved. 

 In science it is the question that matters : the solution 

 can always be found. 



The recrudescence of interest in the capillaries 

 occurred suddenly, many observers being led to the 

 subject by the most diverse considerations. Among 

 these Ebbecke was perhaps the most directly interested 

 in the capillaries themselves. H. H. Dale was led 

 to infer independent changes in the capillaries from 

 his observations on the effects of histamine. Krogh, 

 continuing his researches on respiration, found it 

 necessary to consider the volume of the capillary 

 circulation required for supplying sufficient oxygen 

 to the working tissues. Then, during the War, the 

 committee appointed by the Medical Research Council 

 to investigate the causation of surgical shock was led to 

 ascribe the main part in the production of this condi- 

 tion to the abnormally dilated state of the capillaries. 

 Thus from all sides the attention of physiologists was 

 focussed on these structures. As a result we can 

 boast of a very large accession to our knowledge 

 not only of the capillaries but also of the factors 

 determining the supply of blood to the tissues under 

 varying conditions. 



NO. 2808, VOL. 1 12] 



The volume under review, by the man who has 

 perhaps done more than any single physiologist to 

 advance our knowledge of the capillaries, not only 

 gives a connected account of our present knowledge, 

 but also adds to this a large amount of original work 

 which has been previously unpublished. Prof. Krogh 

 starts with an account of the anatomy and distribu- 

 tion of the capillaries. He shows that in muscle, 

 for example, the number of capillaries which are 

 open varies from time to time according to the activity 

 of the muscle. In a mu'^cle of the horse there are 

 about 1350 capillaries in every square millimetre of 

 transverse section. The transverse section of an 

 ordinary pin is about half a square millimetre. We 

 get an idea of the extraordinary subdivision of the 

 blood supply within a working tissue when we con- 

 sider that within a structure of the size of a pin there 

 are 700 parallel tubes carrying blood, in addition 

 to about 200 muscle fibres. In smaller mammals, 

 such as a guinea-pig, the maximum number of capil- 

 laries per square millimetre is about 4000. This 

 means that an enormous surface of blood is available 

 for interchange to take place with the tissue cells. 

 Krogh makes the following calculation : " Supposing 

 a man's muscles to weigh 50 kilograms and his capil- 

 laries to number 2000 per square millimetre, the total 

 length of all these tubes put together must be some- 

 thing like 100,000 kilometres or two and a half times 

 round the globe, and their total surface 6300 square 

 metres." 



The author makes a plea for further work on these 

 lines. There is a rich field for the anatomists in such 

 quantitative anatomy, especially if the problems 

 attacked are chosen according to their importance 

 for the normal functions of the body. 



Krogh then shows by various means that the 

 capillaries are endowed with an independent power 

 of contractility, and that this is due to the existence 

 of special kinds of muscle cells present in all capillaries 

 and apposed to the outside of their thin endothelial 

 wall. It is noteworthy that these cells were described 

 so long ago as 1873 by Rouget, but the observation 

 was disregarded and soon forgotten. 



In the following lecture the author deals with the 

 innervation of the capillaries. Here again histo- 

 logists long ago described a rich supply of fine non- 

 medullated nerve fibres, but the physiological signifi- 

 cance of these fibres has been revealed only in the 

 last few years. The innervation of the capillaries 

 is of two kinds. In most cases stimulation of the 

 sympathetic provokes contraction. They are also 

 under the influence of the antidromic impulses, which 

 cause dilatation, and, as Bayliss has shown, can be 

 excited in the sensor>' fibres of the posterior root 



