845 



cardiac venous trunk or coronary sinus. This is the ordinary con- 

 dition. Further subdivisions arise, depending on peculiarities of the 

 vena cava itself, which are rare; of the azygos system, which are 

 exceedingly numerous ; and of the coronary vein and sinus, which 

 are again uncommon. Transposition occasionally produces a further 

 modification, in which the superior cava is found on the left side ; 

 whilst the coronary sinus, the oblique vein and the vestigial fold of 

 the pericardium, exist on the right. 



b. In another group there might exist a right vena cava superior 

 and a left vena azygos, as in the Sheep ; but no example of this pos- 

 sible variety has yet been met with in the human subject. 



c. In the third group a right and a left superior cava coexist, as 

 in the Elephant, constituting what is term.ed a double vena cava su- 

 perior. Thirty examples of this condition are adduced, of which 

 eleven only have occurred in adult and otherwise perfect hearts. 

 One of these was met with by the author, and is specially described. 



Lastly, a separate or second class consists of those cases in which 

 the cross branch is wanting, and which are, accordingly, destitute of 

 the characteristic mammalian type, and present, as in Birds, the per- 

 sistent condition of four independent lateral venous trunks. 



The paper is illustrated by original drawings, of the development 

 of the veins in the Sheep and in Man, of the vestiges of the left pri- 

 mitive vein ordinarily found in the adult human subject, and of the 

 fresh example of double vena cava superior in Man met with by the 

 author. 



7. "AMathematicai Theory of Magnetism." By William Thomson, 

 M.A., F.R.S.E., Fellow of St. Peter's College, Cambridge, and Pro- 

 fessor of Natural Philosophy in the University of Glasgow. 



The theory of magnetism was first mathematically treated in a 

 complete form by Poisson, , Brief sketches of his theory, with some 

 simplifications, have been given by Green and Murphy in their works 

 on Electricity and Magnetism. In all these writings a hypothesis 

 of two magnetic fluids has been adopted, and strictly adhered to 

 throughout. No physical evidence can be adduced in support of 

 such a hypothesis ; but on the contrary, recent discoveries, especially 

 in electro-magnetism, render it extremely improbable. Hence it is 

 of importance that all reasoning with reference to magnetism should 

 be conducted without assuming the existence of those hypothetical 

 fluids. 



The writer of the present paper endeavours to show that a com - 

 plete mathematical theory of magnetism may be established upon 

 the sole foundation of facts generally known, and Coulomb's special 

 experimental researches. The positive parts of this theory agree 

 with those of Poisson's mathematical theory, and consequently the 

 elementary mathematical formulae coincide with those which have 

 been previously given by Poisson. 



The paper at present laid before the Royal Society is restricted to 

 the elements of the mathematical theory, exclusively of those parts 

 in which the phenomena of magnetic induction are considered. 



