732. The CEREBRUM IS divided into two hemispheres, by a cleft, or fissure. Into this cleft dips a portion of the dura mater, called the _falx cer´e-bri_, from its resembling a sickle. The apparent design of this membrane is to relieve the one side from the pressure of the other, when the head is reclining to either side. Upon the superior surface of the cerebrum are seen undulating windings, called _con-vo-lu´tions_. Upon its inferior, or lower surface, each hemisphere admits of a division into three lobes--the anterior, middle, and posterior. (Fig. 122, 123) [Illustration: Fig. 121 A section of the skull-bones and cerebrum. 1, 1, The skull. 2, 2, the dura mater 3, 3, The cineritious portion of the cerebrum. 4, 4, The medullary portion. The dark points indicate the position of divided blood-vessels. 5, 5, The lateral ventricles.] -=-=-=-=-=-=-=-=-=-=-=-= 732. How is the cerebrum divided? What is the use of the falx cerebri? What is seen upon the superior surface of the brain? Its inferior? -=-=-=-=-=-=-=-=-=-=-=-= 733. When the upper part of the hemispheres is removed horizontally with a sharp knife, a centre of white substance is brought to view. This is surrounded by a border of gray, which follows the depressions of the convolutions, and presents a zigzag outline. The divided surface will be seen studded with numerous small red points, which are produced by the escape of blood from the division of the minute arteries and veins. The gray border is called the cortical, or _cineritious_ portion, while the white central portion is called the _medullary_. The two hemispheres are connected by a dense layer of transverse fibres, called _cor´pus cal-lo´sum_. 734. In the interior of the brain there are several cavities, two of which are of considerable size, and are called the lateral ventricles. They extend from the anterior to the posterior part of the brain, and wind their way into other parts of the cerebral organ. _Observation._ In the disease called "dropsy of the brain," (hydrocephalus internus,) the serum, or water, is usually deposited in these ventricles. This is effused from the many small blood-vessels of the membrane in these cavities. 735. The brain is of a pulpy character, quite soft in infancy and childhood; but it gradually becomes more and more consistent, and in middle age it assumes the form of determinate structure and arrangement. It is more abundantly supplied with blood than any organ of the system. No lymphatics have been detected, but it is to be presumed that they exist in this organ.