CAPILLARY BLOOD-FLOW 



21 



inclinationem obscuram, secundum ductum 

 ventriculi dextri, et quasi sese leviter 

 contorquere ', &c. 



Obs. 11. In regard to the pericardial 

 pressure eifects the following may be con- 

 sulted : Fran9ois-Franck, Marey's Travaux 

 de Lahoratoire, 1877, p. 107; Cohnheim's 

 Allgemeine Pathologie, 2nd edit., vol. i, 

 p. 21, 1882; W. S. Lazarus-Barlow, Gen- 

 eral Fathology, pp. 49-53, London, 1898 ; 

 T. Lewis, Jnl. of Physiol. 1908, vol. xxxvii, 

 p. 213 ; Yas Kuno, ihid. 1917, vol. li, p. 221. 



Ohs. 13. E. and E. H. Weber, in 1845 

 (Omodei, Annali di Medicina, Naples), dis- 

 covered the inhibitory action of the vagus 

 on the heart. Having noted that voltaic 

 currents applied to the central nervous 

 system of the frog sometimes slow the 

 heart-beat, they sought for the nerve-trunk 

 by which this effect should be transmitted, 

 and found it in the vagus. The inhibitory 

 action of nerves was at that time unknown, 

 and, even after the Webers' publication 

 of their discovery, that a nerve distri- 

 buted to muscle should when stimulated 

 not only not excite it but should arrest its 

 activity appeared at first incredible to many. 

 Volkmann, it seems, had met the pheno- 

 menon in the frog two years earlier, but 

 had disregarded it, believing it a fallacy of 

 experimentation. The first mammal to 

 which the Webers extended their discovery 

 was the cat, and the part of this exercise 

 which deals with the vagus repeats virtually 

 the first mammalian observation by the 

 Webers (Wagner's HandwOrterh. d. Physiol. 

 iii. 2. 32, 1846), except that the exercise uses 

 as stimuli induction currents instead of 

 voltaic, and observes the cardiac effect by 

 inspections of the exposed heart instead of 

 by feeling and counting the pulse. 



Ohs. 14. Harvey used a simple lens for 

 his work on the development of the embryo, 

 and in his day the compound microscope 

 had not come into use ; his death, aetat. 80, 

 occurred eight years before the appearance 

 of Hooke's Micrographia, and four years 

 before the discovery of the capillaries. 

 With no means of seeing actually how the 

 blood passes from the arteries into the veins, 

 Harvey had inferred the existence between 

 them of communicating ' pores '. Descartes, 

 in the Biscours de la Methode, 1637, where 

 he refers to Harvey's discovery, writes : 

 'petits passages aux extrt^mit^s des arteres 

 par oil le sang qu'elles re9oivent du coeur 

 entre dans les petites branches des veines.' 

 Marcello Malpighi, of Bologna, discovered 

 (1661) the communication to be in fact 

 formed by microscopic tubelets, the capil- 

 laries. The average length of a capillary is 

 only 2 mm., so that the reality was not 

 much different from Harvey's presupposition. 

 Malpighi first told of his discovery of the 

 blood-capillaries in a letter to Borelli, the 

 physiologist, at Pisa (De Pulmonibus, Epist. 

 ii ; Opera, tom. ii, London, 1686), describing 

 them in the frog's lung, and prefacing his 

 letter with the quotation 'magnum certum 

 opus oculis video '. In 1672, in a letter to 

 the Eoyal Society, London, he described 

 the streaming of the blood as seen by the 

 microscope from arteries and veins through 

 the capillaries of the chick embryo of the 

 third day [De ovo incubato ; Opera, tom. i, 

 London, 1686). Loewenhoeck, of Delft, was 

 the first to use for such observations the web 

 of the frog's foot (Epistolae ad Soc. Regiam 

 Anglican., Louvain, 1719, p. 155. Letter 

 122, Jan. 1700), the observation which you 

 repeat in § x above. 



