Genesis of Nerve Cells
The nerve cells, while presenting many of the general characteristics described before, exhibit several modifications in details which will be considered furtheron. As, however, all cells have originated from cells similar to one another, it may be interesting to inquire into some of the factors which have determined the main peculiarities of the nervous tissues. It has already been pointed out that protoplasm is sensitive, i.e. has the power of responding to stimuli of various kinds. In the simplest forms of animal life the cells set apart to subserve the nervous functions are essentially chosen for their high degree of sensitiveness to external stimuli, and their power of rapid response to such stimuli. As far as can be judged the varieties of stimulation to which they best respond are those of touch, temperature, and pain.
Naturally they are cells on the surface of the organism, where they are in direct contact with changes in environment. In these simple animals they preside over the preservation of the animal's existence ensuring a supply of nutrition and providing for efficient defence from noxious external forces. In animals higher in the scale of organisation, the differentiation of sex in individual animals introduces a new sphere of nervous activity.
The simple mechanism of a single cell receiving and responding to a stimulus is soon replaced by a double arrangement - one cell receives and transmits the impulse while the other responds. The receiving cell recedes from the surface for protection, leaving, however, a long filamentous portion of its protoplasm to collect and convey the stimuli (Fig. 4); another filament connects it with the responsive cell.
As animals increase in size and complexity it becomes essential that each receiving cell should be in communication with several responsive organs. The nervous system is therefore ultimately represented by two series of cells. The cells of one series have long processes stretching to the periphery for the reception and conveyance of impulses ; the cells of the other series have processes reaching down to the responsive mechanisms - muscles, glands, etc.
The two series are not in actual continuity but are in intimate contact with one another.
Human nervous system
The Animal Cell
Genesis of Nerve Cells
Nerve Cells and Nerve Fibres
General Construction and Development of the Nervous System
The Spinal Cord
The Chief Fibre Systems of the Cerebro-Spinal Axis
The Areas of Localisation on the Cerebral Cortex
The Sense Organs
Human Brain Anatomy
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