A Text-Book of the Principles of Animal Histology (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...Some faint indication of it is shown in one of her figures. This edge shows that the cells of this layer have some very peculiar nervous function besides their connection with the visual cells as described by Miss Hyde and as weakly shown by certain fibrils in the figures of Patten and Hesse. The edge is drawn out into thick groups of rodlike processes, which converge toward an outside central point and join the upper branches of the optic nerve. In doing this, it can be seen, they come to act as intermediate cells between the visual cells and the central ganglion. These visual cells also have a direct connection by means of the lower or lateral branches of this nerve, so that there are two different pathways for the impulse, and we have a case where communicatory nerve cells have entered the retina as they had begun in the worm, Vanadis, where they formed a ganglion below the retina and outside of the eye. This layer of cells probably has some function to perform which is analogous to that of the layers of ganglion cells in the eye-stalk of the Arthropoda or the ganglion layers of the vertebrate retina. Beneath the retina, and separated from it by the wide space filled with the vitreous fluid, is the tapetum, a layer filled with refracting granules and used apparently to reflect the light. This layer is formed by a single, wide, thin cell in other pectens, and the same will probably be found true in the form we are studying. Underneath the tapetum is the pigment layer, a simple epithelial covering whose thick cubical cells are filled with red pigment granules. The dibranchiate cephalopod eye is probably one of the most complex in existence, although this complexity is more easily understood and more superficial than in the vertebrate eye. The eyes of both ...