The medulla is about 1 inch long, conical in shape, being broader above, and connects the pons with the cord. In direction it is vertical, and it ends about the foramen magnum, the ventral surface lying on the basilar portion of the occipital bone, while the dorsal and lateral surfaces are largely covered by the cerebellum. The median furrow on the ventral surface commences as a blind depression, the foramen caecum, at the lower border of the pons, and ends at the decussation of the pyramids. The posterior median furrow commences about half-way down by the approximation of the boundaries of the fourth ventricle. From the antero-lateral furrow the root branches of the twelfth nerve emerge, while from the postero-lateral furrow emerge those of the ninth, tenth, and eleventh nerves. Between the anterior median and lateral furrows lies the pyramid containing the motor strands, which break up lower down into direct and crossed pyramidal tracts, the former going directly down the anterior column of the cord, while the latter crosses at the decussation of the pyramids, to enter the crossed pyramidal tract in the postero-lateral portion of the cord. The lateral surface of the medulla presents the olive lying outside the pyramid, which is formed by the underlying inferior olivary nucleus. The posterior surface presents the funiculi gracilis and cuneatus, in which the columns of Goll and Burdach respectively terminate in the gracile and cuneate nuclei. External to these lies the tubercle of Rolando, formed by the substantia gelatinosa Rolandi, which caps the posterior horn coming to the surface, while at the upper extremity of this posterior surface is the restiform body, which forms the inferior cerebellar peduncle, and in which the direct cerebellar tract runs to the cerebellum. The medulla contains the nuclei of all the cranial nerves after the fourth ; the cardiac, respiratory, and vasomotor centres ; those for vomiting, deglutition, etc. ; those governing the sweat, lachrymal, and salivary secretions, and centres for winking and dilatation of the pupil.
The cord, is partially divided by anterior and posterior median clefts, the former being generally much shallower and broader than the latter, and containing a fold of pia mater not presented by the latter. There is no antero-lateral sulcus along the line of emergence of the anterior nerve roots, but the postero-lateral sulcus from which the posterior roots emerge is marked. The columns of G0ll and Burdach occupy the posterior surface between the median fissure and posterolateral sulcus, the former placed internally to the latter, from which it is separated in the cervical region by the posterior paramedian groove containing a process of pia mater. The grey matter in each half forms a crescentic mass with a blunt anterior smd long posterior horns, the two halves being connected by a grey commissure containing the central canal, an anterior white commissure lying in front of the grey one. The anterior cornua contain the large multipolar nerve cells from which the motor nerves originate, and which form their trophic centres, just as the cells in the cortex govern the motor tracts in the cord. The posterior cornua receive the sensory fibres, which have already passed through a ganglion before entering the cord. Clarkz's column consists of a cell group situated in the posterior horn of grey matter in the dorsal region. The substantia gelatinosa Rolandi is a V-shaped mass capping the posterior horn in the cervical and dorsal regions. The course of sensory fibres is doubtful. Probably many first enter Burdach's column and divide into a short descending and long ascending fibre. The latter is gradually displaced inwards by fibres from other nerves, until it enters Goll's column and ends ultimately in the medulla. Other fibres probably enter Clarke's column, possibly first crossing to the opposite side, whence fibres pass to the direct cerebellar tract and Gower's comma tract (two superficial tracts lying, the former postero-laterally, and the latter antero-laterally). The direct cerebellar tract runs to the cerebellum, while Gower's tract probably does likewise, after passing through the formatio reticularis. The sensory tracts, after passing through the medulla and pons, occupy the tegmentum of the crus, and the posterior third of the posterior limb of the internal capsule, and then pass to the cortex, particularly of the occipital region, through the corona radiata. The direct pyramidal tracts descend on either side of the median anterior fissure, while the crossed pyramidal tract lies in front of the posterior horn. The motor tracts run from the cortex of the Rolandic area through the corona radiata, anterior two-thirds of the posterior limb of the internal capsule, crusta of the crus, pons, and medulla, where most of the fibres cross to enter the crossed pyramidal tract, while a few go direct into the direct pyramidal tract.
Concussion of the brain consists of a molecular vibration of the brain substance, with or without laceration, but with multiple punctiform ecchymoses. The condition may be accompanied by grave symptoms at the time, or they may only appear at a later period. Concussion is a frequent result of railway accidents.
Compression may be caused by depressed fracture, haemorrhage, abscess, tumour, etc., and its effects will depend on the extent and location of the lesion.
Abscesses of the brain occur most frequently in the temporo-sphenoidal lobe or in the cerebellum, the infection being conveyed from the middle ear in many instances. It is noteworthy that cerebral abscesses are generally accompanied by low temperature and slow, full pulse, owing to the compression of the brain.
Of the tumours of the brain, tubercle, if it may be called a tumour, is the most common, the lesions occurring most frequently about the base, although they may occur at any part. Syphilomas occur not infrequently, while of true tumours glioma is the most common. Some gliomas present sarcomatous elements, and may ultimately erode and perforate the skull, and present on the surface as pulsating tumours. As tumours grow comparatively slowly, as a rule, the brain accommodates itself to the increasing pressure for a considerable period, pressure symptoms only becoming marked when the tumour is getting large. In such cases, even when, from size or location, removal of the tumour is out of the question, the raising of an osteoplastic skull flap, so as to relieve the pressure, is frequently followed by marked benefit. Where, on the other hand, the tumour has invaded some focal area, such as the motor cortex, the patient may be subject to fits of Jacksonian epilepsy, which differs from ordinary epilepsy in many important particulars, and which frequently affords valuable information as to the seat of the lesion, from the fit beginning in, and sometimes even being confined to, the part (e.g. a thumb) supplied by the affected portion of cortex. In operating on intracranial tumours, it is usual, instead of trephining, to raise an osteoplastic flap of superficial soft tissues and bone in one piece, so as to expose a considerable surface. After the operation is finished, the osteoplastic flap is replaced.