THE, MEDULLA OBLONGATA. AND SPINAL CORD.
These parts of the cerebro spinal axis are so intimately associated in general structure, and in the continuity of the tracts of grey and white columns of which they are consti tuted, that it will be convenient to describe them together. The medulla oblongata is properly' the upper portion of the spinal cord, in which there is a rearrangement of its columns and the addition of important nerve centers. The spinal cord possesses the distinction, that in its structure the vesicular or grey matter is enclosed within the white matter, while in that of the brain the gra.y matter is external.
The cord including the medulla is about eighteen inches in length, extending from the lower border of the pons \Taro dowmvard the spinal canal to about the level of the lower border of the first lumbar vertebra. It terminates in a slender filament, the ilium terminale, which contains a small amount of gray matter and ends at the sacral canal.
The spinal cord is rounded in transverse sectiou, is divided into lateral halves by, an anterior and posterior median fissure, the former extending upward to the lower border of the pons Varolii, and the latter to the separation of the posterior col umns of the cord which form the sides of the fourth ventri cle. About an inch and a half below the inferior border of the. pons the anterior median fissure is partially obstructed by a tract of decussating fibres, which are those of the anterior py-ramids of the medulla oblongata. The lower extremity of the decussation of the py-ramids is the line of division between the spinal cord proper which is the lower portion, and the medulla oblongata above. At the point of decussa tiou there is an interruption in the grey elements of the spinal cord, produced by- tracts of white fibres, which are directed forward from the deep fibres of the lateral columns of the cord, to become superficial in front of the medulla oblongata. The lateral halves of the cord are united in the middle by a commissure, which is white in front and grey behind. Iu the middle of the grey- commissure is a minute
canal, the ventricle of the spinal cord, which extends through its length and opens into the fourth ventricle at the apex of the calamus scriptorius. The fourth ventricle is a continuation of the central canal of the spinal cord, and is formed by- a defect of the posterior commissure of the cord and a. separation of its posterior columns into the resti form bodies. The fourth ventricle begins in the middle of the posterior surface of the medulla oblongata.
The grey substance of the spinal cord is arranged in two columns, one on each side, united by a grey commissure which contains the central canal. The a.nterior portion of each column is called its anterior horn and is the motor vesicular column of the cord ; the posterior projection or column of grey matter is called the posterior grey horn, or sensory arm. At the junction of the posterior cornu with the grey commissure is a slender vesicular column, called the vesicular column of Clarke. This column is prob ably the cerebellar element of the spinal cord, since it is connected directly with the cerebellum by the direct cere bellar tract, and degenerates along with it in the disease known as cerebello spinal ataxia.
The anterior vesicular columns of the spinal cord are dis turbed or interrupted by- the decussatioa of the anterior pyra raids of the medulla about an inch and a half below the pens Varolii. As the vesicular columns of the cord approach the fourth ventricle the posterior corona, or posterior vesicu lar columns, gradually diverge, and the anterior cornua ap proach each other toward the median line. In the floor of the fourth ventricle the grey columns are spread into a grey sheet, which forms the fasciculi teretes of the floor of the ventricle; the anterior columns lying along the median fissure, and the posterior along the sides of the floor of the ventricle.