Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/52

Rh 42 PHYSIOLOGY [NERVOUS SYSTEM. clays when little or nothing was known of the physiology of the nervous system. Consciousness has been driven step by step upwards until now it takes refuge in a few thousand nerve-cells in a portion of the grey matter of the cortex of the brain. The ancients believed that the body partici pated in the feelings of the mind, and that, in a real sense, the heart might be torn by contending emotions. As science advanced, consciousness took refuge in the brain, first in the medulla and lastly in the cortex. But even supposing we are ultimately able to understand all the phenomena chemical, physical, physiologi cal of this intricate ganglionk mechanism we shall be no nearei a solution of the problem of the connexion between the objective and subjective aspects of the phenomena. It is no solution to resolve a statement of the phenomena into mental terms or expressions and to be content with pure idealism ; nor is it any better to resolve all the pheno mena of mind into terms describ ing physical conditions, as in pure materialism. A philosophy that recognizes both sets of phe- Fir, 31.-.U muscle; SK., skin ; nomena, mutually adjusted W ^SK SH fSSftSSE ever interacting, may be no ex- general sensation ; S.SE., centre of planation ; but at all events it is special sensation; i .SQ., sense of equ unpretentious, recognizes facts, lihrium; VOL., volitional centre; EM., emotional centre ; ID., ideational cen- and does not delude the mind by tre ; EY., eye; EA., ear, TA., taste ; offering a solution which is no solution at all. But aart from S.V., smell; V., vessel; G., gland; H&V., heart and vessels ; EL.O., elec tric organs in some fishes. The arrows .i ,,. , ,f ,1 trie organs in some usiies. iiiearruwi the ultimate question there is the indicate direction of currents, by fol important one of whether phy- lowing which the influence of one siologists are on the whole right cen tre over another may be studied, in relegating sensation or consciousness entirely to the grey matter of the brain. The facts of comparative physiology are against such an exclusive notion, because we cannot deny consciousness to many animals having rudimentary nervous systems. As already said, research in anatomy and physiology and the observation of disease have driven physiologists to adopt the view that the brain is the organ of sensation. This is no doubt true in the sense that it ultimately receives all those nervous impressions that result in consciousness ; but the parts transmitting the nervous impressions are in another sense as much concerned in the produc tion of conscious states as the brain. This view of the matter, put forward by Professor John Cleland in 1870, has not received from psychologists the attention it deserves. His thesis is &quot; that the consciousness extends from its special seat so far as there is con tinuity of the impressed condition ; that when an irritation is applied to a nerve-extremity in a finger or elsewhere the impression (or rather impressed condition) travels, as is generally understood, but exists for at least a moment along the whole length of the nerve, and that as soon as there is continuity of the impressed condition from finger to brain the consciousness is in connexion with the nerve and is directly aware of the irritation at the nerve-extremity &quot; (Evolution, Expression, and Sensation, Glasgow, 1881, p. 106). This view is quite consistent with all the facts of nervous physio logy and presents fewer difficulties than the one generally held, which drives consciousness into the recesses of the nerve-cells in the cortex of the cerebral hemispheres. It appears to keep clear of the prevailing error in the philosophy of modern physiology, that of regarding the body and even the nervous system as a vast series of almost independent organs, losing sight of the community of function and interdependence of parts, characteristic of the body of one of the higher animals. CIRCULATION IN THE BRAIN. Circula- A due supply of healthy arterial blood and the removal of venous tion in blood are essential to cerebral activity. The brain is contained in brain. an osseous case of which the total capacity is variable. The cere bral substance undergoes almost insignificant changes of volume even under a pressure of 180 mm. of mercury. The quantity of blood in the cranium may vary. In the rabbit not more than 1 per cent, of the total quantity of blood of the body (equal to about 5 per cent, of the total weight of the organ) is present at any one time in the brain, whereas in the kidney, by weight, the blood may amount to nearly 12 per cent., and in the liver to as much as nearly 30 per cent. (Foster). If a small round window be made in the cranium and a suitable piece of glass fitted into it, the veins of the pia mater may be observed to dilate or contract if inter mediate pressure be made on the veins of the neck. There is evidently, then, within the cranium some arrangement by which such variations become possible. This is probably accomplished by the anatomical arrangements of the sub-arachnoid spaces. These spaces, containing fluid, communicate freely with each other and with the space surrounding the spinal cord, so that when the quan tity of blood increases in the cranium a corresponding quantity of fluid escapes into the spinal space, the walls of which are no t inextensible like those of the cranium. In young children, before the fontanelles are closed, the variations of circulation and blood- pressure cause pulsations, of which there are two kinds those coinciding with the ventricular systole, produced by the pulsation of the arteries at the base of the brain, and those coinciding with expiration. Pressure on the brain-substance beyond a limit leads to paralysis, unconsciousness, and death. The large sinuses prob ably assist in equalizing internal pressure, and, as inspiration favours the flow of blood from the sinuses, too great distension of these is also avoided. Vaso-motor nerves regulate the calibre of the arteri- oles of the brain, but we know nothing of the conditions affecting the nerves. Nor do we know how the waste-products of the brain are got rid of. There are no lymphatic vessels, but there are spaces around many of the vessels. These probably communicate with the cavities in the membrane containing the cerebro-spinal fluid, the value of which, as suggested by Foster, &quot;depends in all prob ability more on its physiological properties as lymph than on its FIG. 32. Injected convolution of cerebrum (Duret). 1, 1, medullary arteries ; 1, group of medullary arteries in fissure between two neighbouring con volutions ; 1&quot;, arteries of system of arcuate fibres ; 2, 2, 2, arteries of grey substance of cortex ; a, large-meshed capillary network situated under pia mater; 6, smaller-meshed capillary network situated in middle layers of cortex ; c, somewhat larger network in internal layers adjoining white sub stance ; d, capillary network of white substance. mechanical properties as a mere fluid. &quot; The grey matter is much more richly supplied with capillaries than the white matter, as seen in fig. 32. CRANIAL NERVES. The general anatomy of these nerves is described under ANATOMY, Cranial vol. i. p. 880 sq., and it remains only to enumerate their functions, nerves. Their deep roots have also been alluded to in treating of the medulla oblongata and the pons Varolii above. 1. The olfactory nerve. The nerve of smell (see SMELL). 2. The optic nerve. The nerve of sight (see EYE). 3. The oculo-motor or third nerve, motor, supplying all the muscles of the eyeball except the superior oblique and external rectus ; it also supplies the circular fibres of the iris and the ciliary muscle (see EYE). 4. The pathetic or fourth nerve, motor, supplying the superior oblique muscle. 5. The trigeminal or fifth nerve. It has three branches : (A) The ophthalmic division of the fifth, or nerve of Willis, is sensory and supplies the skin of the forehead, the eyebrow, the upper eyelid, the root and lobule of the nose : (I) the palpebral and ocular conjunctiva, the mucous membrane of the lacrymal passages, the frontal sinuses, the upper part of the nasal mucous membrane ; (c) the cornea, the iris, the choroid, and the sclerotic ; (d) the periosteum and bones of the frontal, orbital, and nasal regions ; and (e) muscular sensibility to the intra-orbital muscles. It also influences the secretion of the lacrymal gland. It contains the fibres from the sympathetic governing the radiating fibres of the iris (see EYE), and also the vaso-motor fibres for the iris, choroid, and retina. It is associated with the otic ganglion. (B) The superior maxillary division of the fifth furnishes sensory branches to (a) the skin of the lower eyelid, ala^ of the nose, upper lip, and skin covering the malar bone ; (1&amp;gt;) the mucous membrane of the nasal, pharyngeal, and palatine regions, the maxil lary sinuses, the gums, the upper lip, and the Eustachian tube; (c) the peri osteum of the bones corresponding to its distribution ; and (d) the teeth of the upper jaw. It furnishes filaments to the nasal and palatine glands, and prob-