448 Lord Rayleigh. On the [Feb. 18, 



The cardiac accelerator fibres arise in the cafc from the first four or 

 five thoracic nerves ; the maximum effect is obtained sometimes from 

 the 2nd and sometimes from the 3rd thoracic nerve ; the 1st and the 

 4th thoracic nerves have in some animals a considerable accelerator 

 action, in others little or none ; the 5th nerve appears occasionally to 

 contain a few accelerator fibres, but further evidence is desirable. 



Taking into account the pilo-motor fibres of the cat and dog, it is 

 seen that the cervical sympathetic arises in these animals from the 

 first seven, and in the rabbit from the first eight, thoracic nerves ; the 

 1st thoracic is, however, less represented in the cervical sympathetic 

 of the rabbit than it is in that of the cat and dog. 



Comparing the rabbit with the cat and dog, as regards sympathetic 

 fibres, which are present in all, it results that in the cat and dog 

 the fibres of any one kind are higher in origin, and in some cases 

 present in fewer spinal nerves, than they are in the rabbit. In accord- 

 ance with this, the 2nd thoracic more frequently causes a movement 

 of the fore-foot in the rabbit than in the other two animals. On the 

 whole, the sympathetic fibres of any one kind appear to be slightly 

 higher in the dog than in the cat. 



The uppermost white ramus communicans arises from the 1st 

 thoracic nerve ; the lowest in the dog and cat arises usually, as de- 

 scribed by Gaskell, from the 4th lumbar nerve ; occasionally, however, 

 the 5th lumbar nerve gives off a white ramus to the sympathetic. 

 Both in the upper and lower regions of the spinal cord, there is 

 satisfactory experimental evidence of efferent sympathetic fibres in 

 those spinal nerves which have white rami, and in those only. This 

 is in agreement with the views of Gaskell. 



In the grey rami, medullated fibres of greater diameter than 4 ft 

 and, perhaps, some of the smaller ones are probably afferent fibres, 

 which pass to the spinal cord by the white rami. 



A comparison of the histological characters and of the reflex effects 

 yielded by various parts of the sympathetic, by the depressor, and by 

 the nervus erigens, affords strong evidence that a considerable 

 number of the medullated fibres of larger diameter than 4 /*, although 

 afferent, are not fibres of general sensibility. 



In the course of the paper the results of previous observers are 

 given and discussed. 



III. " On the Relative Densities of Hydrogen and Oxygen, II." 

 By LORD RAYLEIGH, Sec. R.8. Received February 5, 



1892. 



In a preliminary notice upon this subject,* I explained the pro- 

 cedure by which I found as the ratio of densities 15 884. The 

 * ' Boy. Soc. Proc.,' vol. 43, p. 356, February, 1888. 



