TIIE BRAIN.
By this term is usually meant all that portion of the ecrebro-spinal axis contained in the cranial cavity.
The human brain is larger and heavier rela tively to the size and weight of the body than the brain of any other animal, with the pos sible exception of some of the smallest birds and mammals. It is larger and heavier abso lutely than that of any other animal except the elephant and larger whales. It is largest in the white races, smallest in some of the Central African savages. it is larger in the male than in the female. The average weight of the adult male brain of the white races of Europe and America has been estimated at from 40 to 50 ounces, of the female at from 43 to 45 ounces. 'Flip brain is much larger• relatively to the size of the body in the infant than in the adult. the brain of the new-born child Are i i ])!-, on an average from 10 to 12 ounces. The brain in creases in weight up to middle life. After sixty it usually decreases somewhat in weight. It is a common idea that large brains are apt, to be associated with unusual intellectual development. Such is not. however. the ease. While the brains of sonic men of great intellectual attainments have proved to be above the average in weight Cuvie•, 64 ounces; Abercrombie, 63 ounces; Goodsir, 5; M nevs—t e brains of other men ap parently equally intellectual have not been above the average in weight, and Thurman reports a ease of a male epileptic whose brain weighed 62 ounces. and Rickuill one from a similar ease which weighed 641,:. ounces. Brains weighing over 60 ounces have also been observed in the insane.
The brain may be subdivided into the medulla oblongata, the pons Varolii, the mid-brain, the cerebellum, the basal ganglia, and the cerebral hemispheres.
The medulla oblongata is the continuation upward of the spinal cord and extends from the lower limit of the pyramidal deeussation below, to the lower• margin of the pons above. The length of the me dulla is about an inch, and its diam eters. which at its lower portion cor respond to those of the cord, increase from below upward.
Externally the me dulla shows the continuation u p ward of the ante rior and of the pos terior fissures of the cord. These fis sures become more shallow as they as cend, the posterior fissure disappear ing at about the middle of the me dulla where the central canal opens into the fourth ventricle. On either side of the anterior fissure is a prominence caused by the anterior pyramid.and to the outer side of the pyramid the bulging of the olivary body may ice seen. The antern-late•al surface of the medulla is also marked by the exit of the sixth to the twelfth (inclusive) cranial nerves. The posterior surface shows two prominences on either side, one next the pos terior fissure known as the (lava, caused by the nucleus gracilis or nucleus of the column of Goll: the other just to the outer• side of the (lava, due to the nucleus euneatus or nucleus of the column of Burdach. The internal structure of the medulla shows considerable resemblance to that of the cord. This is especially true of the lower part of the medulla. the structures of which are directly continuous with those of the cord. The fibre tracts of the cord. however. as sume, in the medulla, new directions, and in do ing so break up the formation of the gray matter. This, together with the appearance of sonic new masses of gray matter and some new fibre bundles. is the main factor in determining the difference in structure between cord and medulla. The internal structure of the medulla can he best understood by tracing into it the structures of the spinal cord. At the junction of medulla and
cord occurs the pyramidal decussation. Above this deeussation the fibres of the main cortico spinal motor tracts lie in two large bundles, one on either side of the anterior median fissure, and as the anterior pyramids. In the decussa tion of the pyramids the majority of thee fibres cross over to the opposite posteroodateral region pass down into the cord the crossed pyramidal tract. the few fibres remaining next the anterior fissure forming the direct pyramidal tract. The crossing fibres of the decussation sep arate off the anterior horns from the rest of the gray matter, while the intermingling of white matter and gray matter is known ts the formrtio reticularis. Shortly above the pyramidal (teens sat ion masses of gray matter begin to appear in the posterior columns. These are known respect ively as the nucleus graeilis or nucleus of the column of Chill and the nucleus or nucleus of the column of thirdach. lo these nuclei terminate most or all of the fibres of the columns of troll and of Mirolach. The nuclei increase in size pari passn with the decrease in the size of the columns, the latter finally entirely disappearing. These nuclei are therefore nuclei of termination for the fibres of these columns and are of the nature of a relay station in the main sensory conduction path. groin cells in these nuclei, axones pass around the cen tral canal—internal arcuate fibres—deenssate in the median line—sensory decossation—and then turn upward as the fillet or lemnisens. 'Flits is the main continuation brainward of the great sensory traet. Sonic axones from cells in the nuclei it not turn upward in the fillet, but, skirt ing the ventral surface of the as the external artilate fibres, to the cerebellum in the restiform body. Of the lateral tracts of the cord, the direct cerebellar tract joins the resti form body and thus reaches the cerotwIlum through its inferior peduncle, while the tract of (:owers continues upward in the lateral region of the medulla, its exact terminations being as yet undetermined. TLe posterimr horns beeoine, in the medulla, separated from the rest of the gray matter, and, becoming broken up, serve as nuclei of termination for the sensory divisions of some of the cranial nerves. As it passes up through the medulla, the central canal, surrounded by an increased amount of gelatinous substance. gradu ally approaches the posterior surface, to open at about the middle of the medulla into the fourth ventricle. Of the new masses of gray matter which appear in the medulla, the largest are the olivary nuclei. These are irregular convoluted masses of gray matter situated in the ventral part of the medulla behind and to the outer side of the pyramids. They extenol from the upper limit of the sensory decussation to the pins. Each olivary nucleus lint a central core of white fibres which cross the median line and pass to the opposite restiform body farming the cerehello olivary tract. To the inner side of the lower part of the olivary nucleus is a smaller mass of gray matter known as the accessory olivary nucleus, while above the main mass of the olive is the superior olivary nucleus. The distinct line of decussating fibres which extends through the centre of medulla and pons is called the median raphe. The nuclei of certain of the cranial nerves form masses of gray matter in the me dulla. These, together with the root fibres of the nerves, will be found described below in connec tion with the cranial nerves.