Английский язык. Учебное пособие для зоотехнических специальностей

but does not affect adjacent neurons. The neuron is the functional unit of the nervous system, which consists solely of neurons in orderly arrangement, they alone performing the nervous functions. Between any two neurons related in function there is a delicate contact, or synapse, which passes nerve impulses in only one direction, from the axone of one neuron to the dendrite of the other. The nerve impulse that passes along a nerve fiber is possibly a chemical change. It requires the presence of oxygen and produces a minute but measurable amount of carbon dioxide and a rise in temperature; an electrical change occurs, but this is far slower than the speed of electricity.

The Spinal Cord and Nerves. The outer or white matter of the spinal cord consists of bundles of medullated fibres connecting various parts of the brain with the nuclei of spinal nerves and adjustor neurons. The inner grey matter contains adjustor neurons and the nuclei of motor neurons; the nuclei of sensory neurons are in the dorsal root ganglia of spinal nerves. If the dorsal root of the spinal nerve is cut, any sensory impulses from the entering fibres fail to reach the cord and brain. Destruction of the ventral root blocks all motor control by fibres in that nerve. The ventral roots

are variously injured or destroyed in infantile paralysis, leading to impairment of muscular function.

Stimulus and Response. Certain responses in animals can be classified readily, but many cannot, because they differ from one another in degree and not in kind. In lower animals a large percentage are invariable, whereas in higher forms the

invariable and involuntary responses.