Book - Comparative Study of the Sensory Areas of the Human Cortex 1
Lecture I. Visual Cortex
The minute anatomy of the visual cortex (region of the oaloarine fissure, sulcus comu lobulus lingualis) has been already explored by several investigators, among whom we may make particular mention of Meynert, Vicq d*Azyr, Gennari, Krause, Hammarberg, Schlapp, KoUiker, et a/. But their very incomplete researches have been performed by such insufficient methods as staining with carmine, the Weigert-Pall method, or that of Nissl with basic anilines — methods which, as is well known, do not suffice at all to demonstrate the total morphology of the elements and the organization of the most delicate nerve plexuses. They led, however, in spite of the difficulties which stood in the way of these first analyrtical attempts, toward a precise differentiation of the visual cortex from other regions of the brain. At the outset two characteristic differences attracted the attention of the first investigators into the structure of the visual cortex: first, the existence of a very thick stratum of granules, subdivided into accessory strata by lamins of molecular appearance; and, second, the presence in the intermediate layers of the cortex of a white lamina formed of meduUated fibres — which lamina may be seen with the unaided eye. This lamina, appearing in cross-section as a white line, has been named, in honor of the writers who first described it, the line of Gennari or Vicq d'Azyr.
For the sake of brevity, we shall omit a detailed description and discussion of the various layers admitted by the authorities on this region ; suffice it to mention in order the eight layers described by Meynert for the human cortex : First, molecular ; the second, layer of small pyramidal cells ; third, layer of nuclei or granules ; fourth, layer of solitary cells ; fifth, layer of intermediate granules ; sixth, layer similar to the fourth, containing nuclei and scattered cells ; seventh, deep nuclear layer ; eighth, layer of fusiform cells. We may also mention the arrangement of layers recently described by Schlapp for the occipital cortex of the monkey :•(!) layer of tangential fibres ; (2) layer of external polymorphic cells ; (8) layer of pyramidal cells ; (4) layer of granules ; (5) layer of small solitary cells ; (6) second layer of granules ; (7) layer poor in cells ; (8) layer of internal polymorphic cells.
The investigations which I have made on the human cortex as well as on that of the dog and cat, by both the Nissl and Golgi methods, have led me to distinguish the following layers : —
1. Plexiform layer (called molecular layer by authors generally and cell-poor layer by Meynert).
2. Layer of small pyramids.
8. Layer of medium-sized pyramids.
4. Layer of large stellate cells.
5. Layer of small stellate cells (called layer of granules by the authors).
6. Second plexiform layer, or layer of small pyramidal cells with arched axon.
7. Layer of giant pyramidal cells (solitary cells of Meynert).
8. Layer of medium sized pyramidal cells with arched ascending axon.
9. Layer of fusiform and triangular cells (fusiform cell layer of Meynert).
You see that we have modified current nomenclature by introducing terms which call to mind cellular morphology. For we believe that such trite expressions as ^molecular layer," "granular layer," must be
Fig. 1. Vertical section of the visual cortex of man, calcarine aniens, stained by Nissl's method — semischematlc. 1. Fleziform layer. 2. Layer of small pyramids. 3. Layer of medium-sized pyramids. 4. Layer of large stellate cells. 6. Layer of small stellate cells. 6. Second plexiform layer, or layer of small pyramids with arched axon. 7. Layer of giant pyramids. 8. Layer of medinm-sized pyramidal cells with ascending axon. 9. Layer of fusiform and triangular cells.
banished oooe for all from scientific language, and they must be replaced by terms which point ont dominant morphological characters in the nerre structures of each layer or some interesting peculiarity relatire to the course and connections of the axis cylinder processes* The number of layers could be easily increased or diminished, because they are not separated by well-marked boundaries, particularly in Nissl*s preparations. Thus the number of layers which I adopt is somewhat arbitrary. By distinguishing, however, nine layers, I have followed a criterion of individualization which seems to me the most convenient and suitable for my exposition of the cortex as a mechanism composed of elements at a certain level which differ in special morphological features from those of neighboring levels. Besides, the number, extent, and size of cells in these layers vary a little in the different median occipital convolutions, as does also the degree of definite nidification, according as we study the convex or concave aspect of the gyri. Our description relates generally to the cortex of the margin of the calcarine fissure, the region where structural differentiation of the visual cortex is most pronounced.
