Excerpt from Modern Medicine, 1900, Vol. 9

One of the principal modifications which has been observed in this class of cells in the different animals is the length of the process which extends to the periphery. In the lower order of ani mals, the hydra, for instance, this is short, and a striking peculiarity is ob served, particularly in the earthworm, or lumbricus. In this little animal the body of the bipolar cell is situated in the integument itself, the peripheral process necessarily, being very short, while the central process, which extends inward, is comparatively long. In another higher order of worm, in the nereis, for instance, Retzius and others have shown that the peripheral process of this bipolar cell is somewhat longer than in the lumbricus. (see Fig. Ascending in the scale of the animal kingdom, the peripheral proc ess of this bipolar sensory cell becomes longer and longer, the cell body being placed deeper and farther within the body, and finally, in mammals, the high est order of the animal kingdom, we find the body of these bipolar sensory cells collected in groups to form the ganglia of the posterior spinal nerve roots, and the corresponding ganglia of the cranial nerves. These bipolar sensory cells (which form the great avenues, and, with the exception of the organs of special sense, the only avenues through which incoming nerve currents reach the cen tral nervous system) in all probability respond not only to certain kinds of stimulation, such as that produced by contact with other bodies (as we are perhaps most likely to think), but, we have reason to believe, are sensitive and respond to nearly all other forms of stimulation that may act upon the body as well. It is well known, for instance, that the earthworm and other animals, although possessing no special sense organs (such as the eye, the ear, and the organs of smell), respond to light, to sound, and to certain odors when brought in contact; with the surface of the body.

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