NERVOUS SYSTEM 37 



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 — smell ; second, optic — sight ; third, oculo-motor, to 

 several muscles which move the eyeball and iris, controlling in 

 part the size of the pupil of the eye; fifth, trigeminal, or tri- 

 facial, to eye, skin of the face, teeth, etc. ; seventh, facial, prin- 

 cipally motor to muscles of face, mouth, tongue, ear, and neck; 

 eighth, auditory, to internal ear— hearing; ninth, glossopharyn- 

 geal to tongue and phars'nx, sense of taste and general sensation ; 

 tenth, vagus, sometimes called the wandering pair, distributed to 

 heart, lungs, stomach, liver, intestines and other abdominal 

 organs.^ 



Spinal cord. — That portion of the cerebrospinal system which 

 extends within the spinal canal from the occipital opening to the 

 sacrum is known as the spinal cord. It weighs about 10.5 ounces 

 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 extending along the superior surface, 

 and another along the inferior surface. 



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 and is composed of motor fibers. The nerve is therefore 

 mixed. 



These nerves emerge from the spinal canal in pairs, one 

 on each side and at each articulation of the vertebrip. They 

 are named cervical, dorsal, etc., according to location in the 

 spinal column. 



The spinal nerves supply, by their superior branches, the 

 skin and muscles of the neck and back with both sensation and 

 motion. By their inferior branches they supply the lower 

 'The fourth, pathetic, nerves, and sixth, alxhtcens, are motor nerves to 

 other muscles of the eyeball, cooperating with the third in moving it. The 

 eleventh, spinal accessor]!, is also a motor nerve to muscles of the head, 

 neck, withers, arm, etc The twelfth, hypoglossal, controls the muscles 

 of which the tongue is largely composed. 



