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THE MAINTENANCE OF THE INDIVIDUAL 



cross from one side of the brain to the other, like the letter "X," 

 so that the control of the left side of the body is located in the centers 

 of the right side of the brain and vice versa. 



The Cranial Nerves 



There are typically ten pairs of cranial nerves in the lower verte- 

 brates and twelve in mammals, arising from different parts of the 

 brain. Of these, four pairs are of particular interest. Three pairs 

 (I, II, and VIII) are concerned with the innervation of the organs of 

 special sense, while the fourth (X) is that great wanderer, the vagus. 



The first, or olfactory nerve (I) receives 

 stimuli from the nose and conveys 

 them to the brain. The second, or 

 optic nerve (II) emerges from the 

 lateral floor of the diencephalon, its 

 fibers more or less completely crossing 

 in the optic chiasma, that lies just 

 anterior to the infundibular outgrowth 

 of the pituitary body already men- 

 tioned. This nerve transfers the im- 

 pulses which are interpreted in the 

 brain as sight. The third pair of cranial 

 nerves associated with a special sense 

 is known as the auditory nerve (VIII), 

 and has the dual function of hearing 

 and equilibration. 



The remainder of the cranial nerves 

 will be omitted from further discussion 

 except the vagus (X), the ramifications of which are more extensive 

 than those of any of the other cranial nerves. The vagus is a 

 mixture of motor and sensory elements, the former supplying muscles 

 of the pharyngeal and laryngeal region, most of the digestive tract 

 and the liver, pancreas, and spleen, the kidneys as well as the heart, 

 and certain blood vessels. The sensory fibers are distributed to the 

 mucous membranes of the larynx, trachea, lungs, esophagus, stom- 

 ach, intestines, and gall bladder. Inhibitory fibers also reach the 

 heart and, in addition, this versatile nerve supplies the gastric and 

 pancreatic glands with secretory fibers. Much of phylogenetic in- 

 terest may be gleaned from a careful comparative study of the 

 distribution of this and other cranial nerves from fish to man. 



Diagram showing the oplic 

 chiasma in man. Note that the 

 crossing is not complete, a con- 

 dition probably related to the 

 binocular method of vision. 



