352 ZOOLOGY 



differ in appearance in that the dorsal has an enlargement 

 (ganglion) containing nerve cells; the ventral has none. The 

 fibres from these two roots combine to form the nerve, but each 

 fibre remains independent throughout. It is known by experi- 

 ment that the fibres of the dorsal root carry impulses toward the 

 spinal cord ("sensory") and those of the ventral root carry 

 impulses from the cord ("motor"). In certain regions the 

 nerves springing from successive segments of the body may have 

 nurnerous interlacing fibres, forming what is known as a plexus. 



368. Cranial Nerves. Those nerves arising from the brain, 

 that is, inside the cranium, are called cranial nerves. There 

 are ten to twelve pairs of these, but they are not of equal mor- 

 phological value, nor are they strictly equivalent to the spinal 

 nerves. Some have dorsal and ventral roots, but a much larger 

 number have only one group of roots, either dorsal or ventral. 

 Some are purely sensory nerves, some are motor, and some are 

 mixed. How these nerves are related to the segments of which 

 the head is believed to be composed is yet an unsettled question. 



The first or olfactory pair arises from the olfactory lobe of the fore-brain ; its 

 fibres, which are purely sensory, are distributed to the lining of the nose, the end 

 organ of smell. 



The second or optic nerve arises from the second division of the brain (thalam- 

 encephalon), consists of purely sensory fibres, and is distributed to the retina 

 of the eye, the end organ of vision. 



The third, fourth, and sixth pairs are purely motor and are distributed to the 

 muscles of the eye. The third and fourth arise from the third division of the brain 

 (mesencephalori). The sixth nerve arises from the medulla, as do the following. 



The fifth (trigeminal) comes from the anterior portion of the medulla (hind- 

 brain) and, like the spinal nerves, has both dorsal and ventral roots. It is largely 

 sensory, supplying the skin of the face, mouth and tongue. Motor fibres pass to 

 the muscles of the jaw. 



The seventh (facial) is largely motor, is distributed chiefly to the muscles of the 

 face and controls facial expression. 



The eighth or auditory is sensory, and is distributed to the inner ear, the end 

 organ of hearing and of equilibration. 



The ninth or glossopharyngeal is a mixed nerve and is distributed to the muscles 

 and mucous membrane of the pharynx and to the tongue. 



The tenth or vagus arises by numerous roots, has both motor and sensory fibres, 

 and is the most widely distributed nerve in the body. Its fibres pass to the pos- 

 terior visceral arches, lungs, heart, stomach and intestines. 



We find the cranial nerves and their nerve endings concerned chiefly with the 

 higher senses, the muscles of expression, and the sensations and activities involved 

 in the fundamental processes of nutrition. 



