. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 222 ELEMENTS OF BIOLOGY of nerve fibres that make up even the most simple of vertebrate brains. A response to a stimulus that follows such an abbreviated path is known as a reflex act and the path over which it travels is termed a reflex arc (Fig. 150). Cranial Nerves. A number of nerves enter directly into the vertebrate brain without any association with the spinal cord. These are termed the cranial nerves. There are ten pairs in fishes ASSOCIATION NEURONS SENSORY FIBER NEURON. RECEPTOR SEN
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 222 ELEMENTS OF BIOLOGY of nerve fibres that make up even the most simple of vertebrate brains. A response to a stimulus that follows such an abbreviated path is known as a reflex act and the path over which it travels is termed a reflex arc (Fig. 150). Cranial Nerves. A number of nerves enter directly into the vertebrate brain without any association with the spinal cord. These are termed the cranial nerves. There are ten pairs in fishes ASSOCIATION NEURONS SENSORY FIBER NEURON. RECEPTOR SENSORY GANGLION MOTOR FIBER MUSCLE Fig. 150.—Scheme of a reflex arc. (After Kiihn: Gntudriss der aUgeweinen Zoologie. Georg Theime, Leipzig.) and Amphibia and twelve pairs in reptiles, birds and mammals. The fibres that compose them are outgrowths from the nerve cells of the brain and nearby ganglia, but in the case of the optic and of the olfactory nerves the developmental origin is from specialized regions of the fore-brain that during development grow out to become parts of the nares and the eye. The cranial nerves are not segmental structures. Some are purely sensory, some are purely motor, and some are made up of both sensory and motor fibres. A tabulation is given showing the name of each cranial nerve, its region of connection with the brain, its distribution and function in Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Buchanan, James William, 1888-. New York, London, Harper & brothers
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