FIG. 9.—SCHEME OF THE VISUAL CONDUCTION PATHS (after C. von Monakow). a, rods and cones; b, rods; c, nuclei of rods; d, bipolar cells for the cones; e, bipolar cells for the rods; f, large multipolar ganglion cells giving rise to the axons of the N. opticus; g, centrifugal axon of a neuron, the cell body of which is situated in the collieulus superior, its telodendron being situated in the retina; h, Golgi cell of Type II, or dendraxon in the corpus geniculatum laterale; i, neuron connecting the corpus geniculatum laterale with the lobus occipitalis, its axon running in the radiato occipito-thalamica (Gratioleti). The visual impulses are indicated by the arrow. (Barker)
FIG. 10.—SCHEMATIC FRONTAL SECTION THROUGH THE OCCIPITAL LOBE ILLUSTRATING MANIFOLD CONNECTIONS IN A SINGLE LOBE (after H. Sachs), v, cornu posterius ventriculi lateralis; f. c, fissura calcarina; b, upper division: i, lower division; coll, sulcus collateralis; s. o. I, sulcus occipitalis superior (fissura interparietalis); s. o. II, sulcus occipitalis medius; s. o. III, sulcus occipitalis inferior; c. a., calcar avis; g. l., gyrus lingualis; g. f., gyrus fusiformis; g. o. s., gyrus occipitalis superior; g. o. m., gyrus occipitalis medius; g. o. i.. gyrus occipitalis inferior; c, cuneus; 1-10, forceps; 11-14, stratum sagittale internum: 15, stratum sagittale externum; 16, stratum calcarinum; 17, stratum cunei transversum; 18, stratum proprium cunei; 19, stratum proprium s. o. I; 20, stratum proprium s. o. II; 21, stratum proprium. s. o. III; 22, stratum proprium, s. coll.; 23, stratum profundum convexitatis. (Barker.)
According to his suggestion, sleep would be due to a separation of the neurons that run from the surface of the body to the brain cortex, because the various neurons had become too tired for further function. As a consequence of fatigue, their terminal filaments would fall away from one another, external sensations would no longer be communicated to the brain, because the peripheral neuron was not connected with the next in the chain. As a further result, the brain, undisturbed by sensations, would be left at rest so far as the body was concerned. Within the brain certain connections through which flow thoughts that would keep us awake, are also supposed on this theory to be broken, and consequently all the nerve cells have a chance to rest, except, of course, those concerned with such very vital functions as heart movement, respiration and peristalsis.
FIG. 11.—ISOLATED CELL FROM HUMAN SPINAL CORD (Obersteiner).
Somehow, these vital neurons obtain their rest in the intervals between the impulses which they send down, just as cardiac cells do between heart beats.
Neurons in Psychic States.—This same explanation would serve for narcosis, that is, for anesthesia, due to chloroform or ether, or any other drug. As a consequence of the effect of the narcotic upon the central neuron, they are brought into a condition resembling fatigue, at least to the extent of breaking their connections with other neurons so long as they are under the influence of the drug. While sensory nerves at the periphery, then, are being stimulated by the cutting of tissues to which they are attached, the message from them does not reach the brain because of a disturbance of the connections in the chain of neurons. Drunkenness illustrates the same phenomenon in a less degree. The effect of the intoxicant upon the central neurons disturbs sensation because it makes the connection much less complete than before, and so it is easy to understand the familiar occurrence of even severe injuries to drunken men without their being aware of them, or at least without their suffering nearly so much as would be the case if they were not intoxicated.