By reflecting on certain facts connected with the respiratory movements, and which seem to derive no explanation from our actual knowledge of the structure of the spinal chord, I was induced to undertake a series of observations with the view of determining, if possible, the relations which appear to subsist between the spinal nerves and the respiratory nervous centres. These observations, however, led me into a more extended inquiry than I at first contemplated, the results of which I now venture to lay before the Royal Society. It is needless to point out the difficulties which attend, not only the minute investigation, but a clear and connected description, of a structure so intricate and delicate as that of the spinal chord. It may be proper however to state, with regard to the contents of this paper, that those facts only which were verified by cautious and repeated examination have been brought forward with confidence; while in cases where the results of my observations were less satisfactory, I have expressed myself with corresponding reserve. Yet, no labour has been spared in order to arrive at correct conclusions. My observations were made, by means of one of Mr. Ross’s finest microscopes, on many thousand preparations of the spinal chord of Man, of the Calf, Sheep, Pig, Dog, Cat, Rabbit, Guinea Pig and Frog, and occupied a period of more than two years.