NERVOUS SYSTEM AND BRAIN. 383 NERVOUS SYSTEM AND BRAIN. Nerve process CEUEHUAI. CORTEX OF AUILT MAN. The terminal branches of the den- drites running to- ward the molecular layer are not visible. Bctz. Tlicse cells a'le found onlj- in the motor cortex, and it is believed that it is the axones of these cells which pass down through the internal cajisule to the cord as the main motor tract. In this layer arc also found cells — cells of Jlartinolti — the dendrites of which pass down- ward, while their axones pass upward to the molec- ular layer, where they turn and run parallel to the sur- face as the superficial tan- gential fibres. Cells of G(dgi t.vpe 11. are also found in this layer. The axones of these cells branch rajiidly and end. in the gray matter in the vicinity of their cells of origin. The fibres of this layer consist of the axones and ...,....■„.. „„,. dendrites of the cells above PVKAMIDAL CELL , ., , ... „ FHo.M A PERPENDicc- described, and of axones from LAH SECTION OF THE cclls ill othcr rcgious which are passing to their termina- tions here. (3) The cells of the third la.yer are fusiform or irregu- lar in shape. They have no apical dendrites, their proto- plasmic [jrocesses coming off irregularly and ram- ifying mainly in this layer. Their axones pass down into the corona radiata. The fibres of this layer consist of axones and dendrites of the cells just described, of the axones of pj'ramidal cells, and of axones of cells in other parts of the ner- vous system which are passing to their termina- tions here. The corona radiata or central core of white matter radiating out into the gray matter con- sists of: (1) Descending fibres, which are axones of the large and small pyramidal cells and of the polygonal cells of the third layer. These axones become medullated and pass ( a ) to other convolutions of the same hemisphere — associ- ation fibres; (li) through the corpus callosum to the opposite hemisphere — commissural fibres ; (c) to the internal capsule as fibres of the descending tracts — projection fibres. (2) Ascend- ing fibres, w'hich are axones of cells situated elsewhere in the nervous system, and which are here passing to their terminations. These fibres are (a) a.xones of cells situated in other convolutions of the same hemisphere — associ- ation fibres; (b) axones of cells situated in the opposite hemisphere, which have come through the corpus callosum — commissural fibres; (c) axones which have come through the internal capsule from cells situated in lower centres — projection fibres. ('iRcrL.Tiox. The arterial supply of the brain comes from two sources, the internal carotids and the vertebrals. At the base of the brain these four vessels form a remarkable anasto- mosis of large arterial trunks which is known as the circle of illis. The arrangement of these vessels can be best appreciated by refer- ence to the accompanying diagram. From this circle there pass ofl' to the brain three main sets of branches: (1) The anterior cerebral, which pass forward in the great longitudinal fissure, curve around the anterior border of the cor- pus callosum, and. running backward on its upper surface, terminate by anastomosing with the posterior cerebral arteries. They supply the under surfaces of the anterior lobes, part of the internal surfaces of the hemispheres, the corpus callosum, the third ventricle, and the
- interior perforated space. Near their origin
from the internal carotids, the two anterior ce- rebrals are united by a short thick trunk, the anterior communicating. (2) The middle cerebral or I^vlvian arteries, which pass outward along the .Sylvian fissures, within which each divides into three branches — an anterior to the anterior lobe, a posterior to the posterior lobe, and a median to the small lobe near the end of the fissure of .Sylvius. (3) The posterior cerebral arteries, which, arising from the basilar, pass backward and outward on the under surfaces of the occii)ital lobes, each terminating in three branches, which pass respectively to the uncinate gyrus, to the temporo-sphenoidal lobe, and to the lingual lobule, cuneus, and occipital lobe. ■Zang flse. Oortp-Co. ■Infcerttf THE AKTERIEB OF TUB BASE OP THE BRAIN. (The right half of the cerebellum and pons Varolii have been removed.) Frou. lo.. frontal lobe; Ant. t-erfb.. anterior cerebral; i^/ss. .S.W., fissure of Sylvius ; Mirl. i-erfh., middle cerebral) Int. cavo., internal carotid: Ant. rlinr.. anterior choroid ; Post, cowniu., posterior communicatiufi" : Past, cbor., posterior Choroid; Ant. spin., anterior spinal; Verteh., vertebral; Mptl. ohl., medulla oblongata; Ot-cifi. lo., oc- cipital lobe ; Long. Uss., longitudinal fissure ; Ant. conimu., anterior conimunii-ating; Corp. call., corpus callosum; Olf.ue., olfactory nerve; Lam. cio.. lamina cinerea : Op. De. , oi)tic nerve; Op.com.. optic commissure: InfuntUh., infundibulum ; Corpo. albi., corpora albicantia : Svp. cereb.. superior cerebellar; P. Vnr. trims, iir.. pons Varolii transverse arteries; liasi., liasilar; .Ant. cereb., anterior cerebellar; ^'ere/j/zn.. cerebellum ; Inf. cereb., inferior cere- bellar; A., circle of Willis. All these arteries ramify in the pia, where they anastomose freely. From this anastomosis are given oil' small branches which enter the substance of the brain, the shorter ones breaking lip into a capillary network in the gray matter, the longer ones passing to the more deeply situ- ated white matter. -fter entering the brain substance, there is no further anastomosis, the arterial ca])illaries passing diiectly into ve- nous capillaries without communicating with the capillaries of neiglilioring arteries. Besides these cortical branches, branches from the circle of Willis supply tJie basal ganglia. Like those in the cortex, these arteries do not anastomose after entering the brain substance. The blood supply of the spinal cord is derived from three main arteries, the anterior spinal and two posterior spinal. The anterior spinals arise from the vertebrals, and, passing downward on the anterior surface of the medulla, unite to form a single trunk, which passes down the
