Page:The American Cyclopædia (1879) Volume III.djvu/198

 192 BRAIN spheres together as their great commissure, and securing their connected action, the fibres pass- ing from one to the other like a bridge ; at its anterior and posterior extremity it is folded downward toward the base of the brain. On cutting a little deeper, an irrregular cavity is opened on each side, the lateral ventricle, con- taining the striated and optic bodies; these cavities are lined by a serous membrane, se- creting a fluid, the undue accumulation of which constitutes hydrocephalus internus, or water on the brain, a fatal disease of children, in which the substance of the brain may be- come almost obliterated, and the bones of the yet ununited skull distended almost to the size of an adult head. The fifth ventricle is the space between the layers of the septum luci- dum, an extension of fibrous matter connecting the anterior reflection of the corpus callosum with the horizontal fibrous stratum called the fornix, and separating the anterior horns of the lateral ventricles. Between the optic thai- ami and striated bodies in the ventricles, in a superficial groove, is the taenia semicircularis, a delicate band of fibrous matter, commissural in its character. The posterior horn of the lateral ventricle, according to Owen, is peculiar to man, as also is the hippocampus minor, a pro- jection of one of the convolutions into it; in its inferior horn is the hippocampus major, and a considerable portion of the vascular choroid plexus. The cerebral hemispheres, after the membranes have been removed, present a pe- culiar folded arrangement of their surface, the FIG. 7. Portion of Eight Hemisphere of the Human Brain, divided horizontally, showing the convolutions, and ar- rangement of white and gray matter. " convolutions ;" the folds consist of a layer of gray matter, varying from one eighth to one quarter of an inch in thickness. Physi- tology has shown that the gray matter of the nervous centres is the originator of nervous force, while the white matter serves only to convey impressions to or from the different parts of the body ; hence the greater the num- ber of these convolutions, or, in other words, the greater the amount of the gray substance, the greater will be the physiological power of the brain. In the rat and the mole the sur- face of the brain is quite smooth ; from these the convolutions increase in number up to man. Their arrangement, though never the same in two brains, nor on opposite sides of the same brain, cannot be supposed to be purely accidental ; there are certain ones always pres- ent (when any exist), whose situation and size influence the disposition of the others ; in man the variable and additional convolutions are chiefly on the top and front of the hemi- spheres. The lower the position of an animal in the scale, and the less developed ' the organ as we approach infancy, the greater is the sym- metry of the two sides. It is said that the convo- lutions in the inferior races of man (Todd and Bowman) present a more symmetrical arrange- ment than is usually found in the more cul- tivated races. If the gray matter of the cere- bral convolutions and the cerebellar layers were spread out, it would occupy about 6TO square inches, which by this admirable ar- rangement are packed into the small extent of the brain. Each convolution consists of a fold of gray matter enclosing a process of the white; the gray matter forms a continuous unbroken sheet over the cerebral surface ; the greater part of the white fibres emerge from the gray matter, and thence converge to the central parts of the brain. The fibres which unite different portions of the same or of oppo- site hemispheres are called "commissures;" the transverse are the corpus callosum, the an- terior, posterior, and soft commissures; the longitudinal are the fornix and the superior longitudinal commissure. The corpus callosum connects the great bulk of the hemispheres, especially at the lower part ; it is wanting in fishes, reptiles, birds, and the lower mammals. The anterior commissure particularly unites the striated bodies, many of its fibres passing through them and radiating to the middle cere- bral lobes; it is very large in the marsupials, which have no corpus callosum. The posterior commissure connects the optic thalami, and is connected with the pineal body. The soft commissure also passes from one optic thalamus to the other, dividing the third ventricle into an upper and lower portion ; unlike the other commissures, it contains gray matter. The superior longitudinal commissure is enclosed in the convolution overhanging the corpus callo- sum, and connects the anterior and middle lobes with the posterior. The fornix or vault is the most remarkable, extensive, and compli- cated of all the commissures; it is situated immediately under the corpus callosum, with which it is closely connected posteriorly; it may be divided along the median line into two