NERVOUS SYSTEM. 33 



arrangement of nerve tissue. It is the function of the cerebellum 

 to control the voluntary muscles that they may work in har- 

 mony. 



The medulla is a continuation of the spinal cord and extends 

 from the occipital opening in the brain cavity, to the pons or 

 bridge and is located posterior and inferior to the cerebellum. 



Within the medulla are located a number of important cen- 

 ters, among them, the ones controlling respiration; the caliber of 

 blood vessels, distribution of the blood according to the needs 

 of the body; the center controlling the processes of swallowing; 

 the vomiting center ; and the center controlling the secretion of 

 saliva. 



The isthmus is that part of the inferior portion of the brain 

 which connects the pons with the cerebrum. 



Cranial nerves. — There are twelve pairs, of which the first is 

 olfactory (nerve of smell) ; 2nd, optic (sight) ; 3rd, occulo motor, 

 to muscles of the eye ball; 5th, trifacial, to eve, skin of the face, 

 teeth, etc. ; 7th, facial, to muscles of face ; 8th, auditory, to internal 

 ear — hearing; 9th, glosso-pharyngeal to tongue and pharynx, 

 sense of taste and general sensation ; 10th, pneumo gastric , some- 

 times called the wandering pair, distributed to heart, lungs, stom- 

 ach, liver, intestines and other abdominal organs. 



Spinal cord is that portion of the cerebrospinal system which 

 extends within the spinal canal from the occipital opening to the 

 sacrum. It weighs about 10.5 oz. and consists of white matter 

 on the outside and gray matter in the interior, and is covered by 

 the same three membranes as the brain. The spinal cord is 

 marked throughout its entire length by two fissures, one extend- 

 ing along the superior surface, and another along the inferior sur- 

 face. 



It is the function of the spinal cord to act as a means of 

 communication between the brain and spinal nerves, and as a 

 reflex nerve center. 



Spinal nerves. — There are about 42 pairs. Each nerve has 

 origin in two roots. One root (superior) comes from the upper 

 portion of the cord and is composed of sensory fibers. The other 

 root (inferior) comes from the lower portion of the cord anil 

 is composed of motor fibers. The nerve is therefore mixed. 



These nerves emerge from the spinal canal in pairs, one on 



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