. Biology and human life. Biology. 220 BIOLOGY AND HUMAN LIFE gland) acting as an effector, or effect- producing structure (see Fig. 109). Nerve cells, which differ from the other cells of the body in their special irritability, have distinct structural peculiarities (see 7, Fig. 31). There are (i) a cell body, which contains the nucleus, and (2) outgrowths, or libers, of two kinds—a long, slender fiber called the axon, and shorter processes that branch irregularly, like a tree, called dendrites (from a Greek word meaning ''tree"). A nerve cell is sometimes called a neuron. Neurons are fo
. Biology and human life. Biology. 220 BIOLOGY AND HUMAN LIFE gland) acting as an effector, or effect- producing structure (see Fig. 109). Nerve cells, which differ from the other cells of the body in their special irritability, have distinct structural peculiarities (see 7, Fig. 31). There are (i) a cell body, which contains the nucleus, and (2) outgrowths, or libers, of two kinds—a long, slender fiber called the axon, and shorter processes that branch irregularly, like a tree, called dendrites (from a Greek word meaning ''tree"). A nerve cell is sometimes called a neuron. Neurons are found in all parts of the body, but the cell bodies are usually crowded together in special groups or regions, while the fibers are grouped into long nerves. The nerve- cell bodies are found chiefly in the gray cortex, or "bark," of the brain, in the core of the spinal cord, and in special groups, or ganglia, in various parts of the body. The single neuron connects with other neurons through a close network formed by the dendrites and the branchings of the axon. It is not cer- tain whether the protoplasm of one cell actually touches the protoplasm of the next in one of these connecting regions, but it is certain that the stim- ulation of one cell can transmit the disturbance to the next through such a connection, which is called a synapse (see s, Fig. 109). In some nerve cells a stimulation is received by the delicate branch-. Fig. III. Behavior limited by nerve connections The diagram shows the nerve connections of a simple mus- cular reflex, with collateral con- nections to the brain. Such connections make possible automatic reflexes as well as voluntary movements. If the afferent nerve is cut, as at a^, only voluntary movement is possible, and there is no sen- sation. If the efferent nerve is cut, as at Cj, neither reflex nor voluntary movement is pos- sible, but sensation remains. If the spinal cord is cut high up, as at a2, e^, neither sensa- tion nor voluntary m
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