Smell. Everything conspires to point to the median descending part of the temporal lobes as being the organs of smell, Even Ferrier and Munk agree on the hippocampal gyrus, v m m FIG. 18. though Ferrier restricts olfaction, as Munk does not, to the lobule or uncinate process of the convolution, reserving the rest of it for touch. Anatomy and pathology also point to the hippocampal gyrus; but as the matter is less interesting from the point of view of human psychology than were sight and hearing, I will say no more, but simply add Luciani and Seppili's diagram of the dog's smell-centre.* Of * For details, see Ferrier's Functions,' chap. IX. pt. III, and Chas. K. Mills: Transactions of Congress of American Physicians and Sur geons, 1888, vol. 1. p. 278. Taste we know little that is definite. What little there is points to the lower temporal regions again. Consult Ferrier as below. Touch. Interesting problems arise with regard to the seat of tactile and muscular sensibility. Hitzig, whose experiments on dogs' brains fifteen years ago opened the entire subject FIG. 19.-Luciani's Olfactory Region in the Dog. which we are discussing, ascribed the disorders of motility observed after ablations of the motor region to a loss of what he called muscular consciousness. The animals do not notice eccentric positions of their limbs, will stand with their legs crossed, with the affected paw resting on its back or hanging over a table's edge, etc.; and do not resist our bending and stretching of it as they resist with the unaffected paw. Goltz, Munk, Schiff, Herzen, and others promptly ascertained an equal defect of cutaneous sensibility to pain, touch, and cold. The paw is not withdrawn when pinched, remains standing in cold water, etc. Ferrier meanwhile denied that there was any true anesthesia produced by ablations in the motor zone, and explains the appearance of it as an effect of the sluggish motor responses of the affected side.* Munkt and Schiff, on the *Functions of the Brain, chap. x. § 14. + Ueber die Functionen d. Grosshirnrinde (1881), p. 50 Lezioni di Fisiologia sperimentale sul sistema nervoso encefalico (1873), p. 527 ff. Also 'Brain,' vol. Ix. p. 298. contrary, conceive of the 'motor zone' as essentially sensory, and in different ways explain the motor disorders as secondary results of the anesthesia which is always there. Munk calls the motor zone the Fühlsphäre of the animal's limbs, etc., and makes it coördinate with the Sehsphäre, the Hörsphäre, etc., the entire cortex being, according to him, nothing but a projection-surface for sensations, with no exclusively or essentially motor part. Such a view would be important if true, through its bearings on the psychology of volition. What is the truth? As regards the fact of cutaneous anesthesia from motor-zone ablations, all other observers are against Ferrier, so that he is probably wrong in denying it. On the other hand, Munk and Schiff are wrong in making the motor symptoms depend on the anesthesia, for in certain rare cases they have been observed to exist not only without insensibility, but with actual hyperæsthesia of the parts.* The motor and sensory symptoms seem, therefore, to be independent variables. In monkeys the latest experiments are those of Horsley and Schaefer, whose results Ferrier accepts. They find that excision of the hippocampal convolution produces transient insensibility of the opposite side of the body, and that permanent insensibility is produced by destruction of its continuation upwards above the corpus callosum, the socalled gyrus fornicatus (the part just below the 'callosomarginal fissure' in Fig. 7). The insensibility is at its maximum when the entire tract comprising both convolutions is destroyed. Ferrier says that the sensibility of monkeys is 'entirely unaffected' by ablations of the motor zone,‡ and Horsley and Schaefer consider it by no means necessarily * Bechterew (Pflüger's Archiv., vol. 35. p. 137) found no anæsthesia in a cat with motor symptoms from ablation of sigmoid gyrus. Luciani got hyperesthesia coexistent with cortical motor defect in a dog, by simultaneously hemisecting the spinal cord (Luciani u. Seppili, op. cit. p. 234). Goltz frequently found hyperæsthesia of the whole body to accompany motor defect after ablation of both frontal lobes, and he once found it after ablating the motor zone (Pflüger's Archiv, vol. 34, p. 471). + Philos. Transactions, vol. 179, p. 20 ff. Functions, p. 375. abolished.* Luciani found it diminished in his three ex periments on apes.† In man we have the fact that one-sided paralysis from disease of the opposite motor zone may or may not be accompanied with anesthesia of the parts. Luciani, who FIG. 20.-Luciani's Tactile Region in the Dog. believes that the motor zone is also sensory, tries to minimize the value of this evidence by pointing to the insufficiency with which patients are examined. He himself believes that in dogs the tactile sphere extends backwards and forwards of the directly excitable region, into the frontal and parietai lobes (see Fig. 20). Nothnagel considers that pathological evidence points in the same direction; ‡ and Dr. Mills, carefully reviewing the evidence, adds the gyri fornicatus and hippocampi to the cutaneo-muscular region in man.§ If one compare Luciani's diagrams together (Figs. 14, 16, 19, 20) one will see that the entire parietal region of the dog's skull is common to the four senses of sight, hearing, smell, and touch, including muscular feeling. The corresponding region in the human brain (upper parietal and supra-marginal gyri-see Fig. 17, p. 56) seems to be a somewhat similar place of conflux. Optical aphasias and motor and tactile disturbances all result from its injury, especially when that is on the left side. The lower we go in the animal scale the * Pp. 15-17. Op. cit. p. 18. + Luciani u. Seppili, op. cit. pp. 275-288. I See Exner's Unters. üb. Localization, plate xxv. less differentiated the functions of the several brain-parts seem to be.* It may be that the region in question still represents in ourselves something like this primitive condition, and that the surrounding parts, in adapting themselves more and more to specialized and narrow functions, have left it as a sort of carrefour through which they send currents and converse. That it should be connected with musculo-cutaneous feeling is, however, no reason why the motor zone proper should not be so connected too. And the cases of paralysis from the motor zone with no accompanying anesthesia may be explicable without denying all sensory function to that region. For, as my colleague Dr. James Putnam informs me, sensibility is always harder to kill than motility, even where we know for a certainty that the lesion affects tracts that are both sensory and motor. Persons whose hand is paralyzed in its movements from compression of arm-nerves during sleep, still feel with their fingers; and they may still feel in their feet when their legs are paralyzed by bruising of the spinal cord. In a similar way, the motor cortex might be sensitive as well as motor, and yet by this greater subtlety (or whatever the peculiarity may be) in the sensory currents, the sensibility might survive an amount of injury there by which the motility was destroyed. Nothnagel considers that there are grounds for supposing the muscular sense to be exclusively connected with the parietal lobe and not with the motor zone. "Disease of this lobe gives pure ataxy without palsy, and of the motor zone pure palsy without loss of muscular sense." He fails, however, to convince more competent critics than the present writer, so I conclude with them that as yet we have no decisive grounds for locating muscular and cutaneous feeling apart. Much still remains to be learned about the relations between musculo-cutaneous sensibility and the cortex, but one thing is certain: that neither the occipital, the forward frontal, nor the temporal lobes seem to have anything essential to do with it in man. * Cf Ferrier's Functions, etc., chap. IV and chap. x, § 6 to 9. + Op. cit. p. 17. Eg. Starr, loc. cit. p 272; Leyden, Beiträge zur Lehre v. d. Localization im Gehirn (1888), p. 72. |