(1.) Galen believed that the medulla spinalis commences as high up as the lower border of the pons Varolii . This, like nearly every other, opinion of the Greek anatomist was adopted by all his successors until the time of Sylvius and Vesalius. Sylvius traced the origin of the spinal chord from the whole base of the brain 1 ; Vesalius, only from the corpora quadrigemina2 . Columbus 3 , Varolius 4 , Spigelius 5 , Laurentius 6 , Riolanus 7 , Highmore 8 , described it as arising by two roots,—one large, from the entire base of the brain; the other small, from the cerebellum. But Piccolhomini introduced a better description of these parts, by restricting the term “ medulla spinalis ” to the contents of the spinal column; while to the whole intra-cranial prolongation along the base of the brain, he applied the term “ medulla oblongata ” 9 , which has been retained to the present day, though many anatomists have differed with regard to the number of parts it has been supposed to comprehend. According to Willis 10 , it includes the whole base of the brain, from the corpora striata —which he called “medullæ oblongatæ apices"—to the foramen magnum . The same view was adopted by Vieussens 11 , Winslow 12 , and others; while Rolando 13 employed the term in a more restricted sense ; and Ridley 14 , who did the same, substituted for it the term “isthmus,” and Chaussier 15 that of “mesocephale,” which included the pons, tubercula quadrigemina , with the superior peduncles of the cerebellum , and not the pons only , as sometimes stated. Flourens 16 , again, on the ground of his experimental inquiries,—which led him to conclude that the seat of excitability begins or ends with the corpora quadrigemina ,—limits the medulla oblongata between these bodies and the eighth pair of nerves. In this country the medulla oblongata is generally understood to extend from near the points of the anterior pyramids to the lower border of the pons , and as such it is described in this memoir. (2.) It is well known that in the spinal chord the columns of white and grey substance are parallel to each other, and preserve the same relative position throughout their entire course; but in the medulla oblongata these parts not only assume a different arrangement, by becoming more or less blended with each other and with new structures, but frequently pursue a curvilinear direction in different planes inclined at varying angles. It is evident, therefore, that transverse and longitudinal sections, however transparent they may be made, must be insufficient, when employed alone, to complete the examination of so intricate a structure. For this reason, after having made myself thoroughly acquainted with the microscopical anatomy of these parts, and traced their relations and connexions, as far as possible, by my own method of preparation, which I find superior to any other that I have seen of the same kind, I then employed, in conjunction with it, the method of Reil; so that when necessary, the structure revealed at different stages of the dissection was compared with transparent sections through the same parts. Great assistance was also obtained by making, simply for my own use, as I dissected through the depth of a part, a multitude of sketches, which were placed side by side and carefully examined in succession: the plan is tedious and demands patience, but car be confidently recommended to anatomists as a valuable expedient for ascertaining the arrangements of a complicated and intricate structure of which the parts are so continually changing their course and relative position, in varying planes, that in the dissection each must necessarily be destroyed whilst seeking its relation to others.