. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Dendrites -^— Cell Body — Neurite (Axon) ^Collateral II. THE STRUCTURAL UNITS As would naturally be expected, the ectoderm, that is, the embryonic tissue which presents primary contact with the environment, gives rise to the principal cell units destined to form the nervous system. These ectodermal cells eventually depart very far from their original epithelial compactness, becoming either true nerve cells (neurons), or non-nervous structures (neuroglia ce
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Dendrites -^— Cell Body — Neurite (Axon) ^Collateral II. THE STRUCTURAL UNITS As would naturally be expected, the ectoderm, that is, the embryonic tissue which presents primary contact with the environment, gives rise to the principal cell units destined to form the nervous system. These ectodermal cells eventually depart very far from their original epithelial compactness, becoming either true nerve cells (neurons), or non-nervous structures (neuroglia cells), which have a secondary supportive role. Still other cells, from the embryonic meso- derm, may also serve the all-important neurons in the form either of nutritive blood or as pro- tective coverings. ' The neuron theory, which is the most com- monly accepted working hypothesis of biologists concerned with the nervous system, holds to two propositions: first, that the nervous system proper is entirely made up of neurons; and second, that transmission of nerve impulses from one neuron to another is by means of make-and-break con- tact without protoplasmic fusion. The point of contact between neurons is called a synapse. The entire working nervous system in man involves several hundred million synapses. After differentiation from their general- ized spherical embryonic form, neurons come to vary enormously in size, shape, thick- ness, length, and manner of their insulation. The cell body immediately surrounding the nucleus sprouts out into various processes, the so-called "nerve fibers," which may become enormously attenuated and lengthened (Fig. 593), although always remaining an essential part of the cell and under immediate control of the nucleus. Thus the neuron assumes a form well adapted to its primary function of transmission at the same time that it is specializing in sensitivity,. Fig. 593. A nerve Please note that these images are extracted from
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