Page:The New International Encyclopædia 1st ed. v. 14.djvu/419

* NERVOUS SYSTEM AND BRAIN. 369 NERVOUS SYSTEM AND BRAIN. the nervous i)if,'aiis eonoorned in sensation, voli- tion, and menial action. The syxiijullutic system consists essentially of a chain of yanirlia connected l)y nervous cords, which extends from the cranium to the pelvis. aloni; each side of the vertehral c(dunin. an<l from which nerves with large ganglionic masses pro- ceed to the viscera and bloodvessels in the cavi- ties of the ehest. abdomen, and pelvis. It was termed by Bichat the nervous system of organic life, since it seems to regulate — almost or quite independently of tile will — the due performance (if ihe functions of the organs of respiration, cir- culation, and digestion. The Centk.^l Nervous System. The brain and spinal cord are covered and protected by three membranes or meninges, as they are frequently Occipla L.sacpUx. VIEW OF THE CEI1EBH0-8PISAL AXIS. The right half of the cranium and trunk of the body havpbpen removed by a vertii-al section : the membranes of tlip ri^ht side of the brain and spinal curd havp been rlpaivii nway, and tlip roots and first part of the fifth and twi-lfth oranial nerves, and of all of tlu- spin.'il nerves of the ri^bt sidp. liavp bppn dissprt^'d out and laid spparately on the w;tll of tlip skull and on the spveral verti'brie opposite to the place (»f their natural exit from the cranio-spiual eavity. Fvun. in., frontal lobp; Temp, lo., temporal lobe; Ofcip. 1(1., ori-ipital lobP : 1'rreh.. cprpbpllum : I'o. Var.. pons Varolii ; .lfil. (thl., nu'dnlla oblongata: .'J and b. upper and loupi- extrpMiitips of the spinal cord; Cau. fti. on tlip last lumbar vprt-pbrul spine marks thp cauda equina ; er. trig., tlip three prini-ipal branches of the nervous tri|i?eminus ; C. 1.ST, the sub-occipital or first cervical nerve; ('. Stti, the eighth or lowest cervical nerve : D. 1st, the first dorsal nprvp : D. 12tb, the last dorsal nerve ; L. Ist. the first lum- bar iiprve ; L. 5th, the last lumbar nerve ; N. Ist, the first sacral nerve; S. 5tb, the fifth sacral nerve; L. aac. plex,, the left sacral plexus; Coc. Be., the coccygeal nerve. termed — viz. the dura mater, the arachnoid, and the pia, mater. The dirra mater is a strong fibrous membrane, which supplies the cranial bones with blood in early life, and adheres firmly to their inner surface. The arachnoid (so called from its being supposed to be as thin as a spi- der's web) is a serous membrane, and, like all serous membranes, is a closed sac, consisting of a parietal and a visceral layer. The parietal layer adheres to the inner surface of the dura mater, wliile the visceral ^ayer somewhat loosely invests the brain and spinal cord, from direct contact with which, however, it is separated by the inter- vention of the piu mater and some loose areolar tissue. The pia mater is an extremely vascular membrane consisting of minute blood-vessels, held together by an extremely fine areolar tissue. It dijis down between the convolutions and fis- sures of the brain, and is prolonged into the interior, forming the velum interp<jsitum and the choroid plexuses of the fourtli ventricle. It is by means of this membrane that the blood-vessels are conveyed into the nervous substance. The C'EnEiiHo-SpiNAL Nervous System. In de- scribing this it is more convenient to begin with a description of the spinal cord, the study of which is essential to a proper appreciation of the higher and more complicated centres. The Spinal Cord is that portion of the cerebro- spinal axis which is contained in the spinal canal. It extends from the upper liorder of the first cervical vertebra or atlas above, to the middle or lower margin of the first luniliar vertelira below. The cord is continuous superiorly with the me- dulla oblongata, while its lower extremity tapers ofi' into a slender cord, the filuni terminale. At two levels, one in the cervical and one in the lumber region, the diameter of the cord is con- siderably larger than elsewhere. These are known respectively as the cerrical and the lum- hur enlargements. The posterior median septum and the anterior median fssure almost divide the cord longitudinally into two symmetrical halves, while the spinal nerve roots leaving the cord at quite regular intervals serve to divide it into segments. These segments correspond in number to the spinal nerves. There are thus eight cerrical, twelve dorsal, five lumbar, five sacral, and one or two cocryyeal segments. The internal structure of the cord can be best appre- ciated by studying a transverse section through the cervical enlargement stained by Weigert's method. Such a section shows that two sub- stances enter into its composition, one situ- ated centrally, the other on the periphery. On account of their ajipearanee in fresh tissues, the former is called firai/ matter, the latter uMte matter. The gray matter is made up mainly of cells and their dendrites, and of non-mediillated axones; the white matter consists mainly of medullated nerve fibres, its whiteness being due to the myelin. The gray matter presents some- what the form of the letter TI. Posteriorly, it is slender, and almost touches the surface — posterior horns — while anteriorly it is broad — anterior horns — and separated from the surface by a considerable layer of white matter. The expanded tip of the posterior horn is known as the head or caput, and contains a ffclatinous sub- stance, the .s»;;.s7(rH/ia gelatinosa of Rolando. The narrow portion connecting the head with the central gray matter is called the neck or ccriiij;. Laterally, the gray matter extends out somewhat into the white matter as the lateral horn or processus reticularis lateralis. The gray horns of the two sides of the cord are connected by a commissure containing the central canal. In foetal life this canal is open and continuous with the ventricles of the brain. In adults it is