NERVOUS SYSTEM 



4120 



NERVOUS SYSTEM 



are in case of the wire. The nerves divide and 

 subdivide as they extend from the main nerve 

 trunk until they form a complete network un- 

 der the skin, so closely woven that you cannot 

 prick it anywhere without injuring one or more 

 of the nerves. 

 The frayed - out 

 ends of the neu- 

 rones take differ- 

 ent forms for dif- 

 ferent purposes. 

 Those connected 

 with the sense of 

 touch, for in- 

 stance* take one 

 form, and those 

 connected with 

 the sense of sight 

 another, so that 

 each set of nerves 

 is especially 

 adapted to its 

 peculiar function. 

 Nerves, like the 

 brain, consist of 

 gray and white 

 matter, the gray 

 matter forming 

 the axis and the 

 white the sheath. 

 There are two 

 classes of neu- 

 rones, those car- 

 rying impulses to 

 the brain and 

 those carrying im- 

 pulses from the 

 brain, and neither 

 set can perform 



A NEURONE 

 From the optic nerve ; very 



the function of 



greatly magnified. 



the other. The nerves formed from neurones 

 of the first class are called sensory nerves and 

 those formed from the second class, motor 

 nerves. 



Cerebrospinal System. The cerebrospinal 

 system comprises the brain, the spinal cord 

 and the nerves branching off from them. It 

 presides over voluntary motion, is the seat of 

 the sense of touch and other senses connected 

 with the muscles, and through one pair of the 

 cranial nerves connects with the sympathetic 

 system, which, in a measure, it controls. 



Spinal Cord. The spinal cord is the largest 

 nerve trunk in the body. It extends from the 

 medulla oblongata through the cavity in the 

 spinal column to the lumbar vertebrae. It is 



a 



SPINAL CORD 



divided by folds into right and left halves, and 

 each half has three columns slightly marked by 

 folds. At its lower extremity it divides into a 

 number of nerve trunks, some of which continue 

 downward in the spinal column to the sacrum; 

 others extend to 

 the lower limbs. 

 In the lower part 

 of the neck there 

 are also enlarge- 

 ments from which 

 nerves branch off 

 to the arms. The 

 spinal cord is 

 composed of gray 

 and white nerve 

 tissue. The gray 

 matter is enclosed 

 in the white, and 



a cross section of 



, A section of the spinal 



it somewhat re- cord (a), showing sensory 



spmhlpa tViP Ipt nerve (b), motor nerve (c), 

 5 embles tne let- and nerve trunk (d) 



ter H. In the 



center of the cord there is a canal which, at its 



upper end, connects with the ventricles of the 



brain. 



Spinal Nerves. Thirty-one pairs of nerves- 

 branch off from the spinal cord and extend to 

 all parts of the surface of the body below the 

 head and neck. Each nerve trunk contains 

 sensory and motor nerves, the sensory branch- 

 ing from the posterior and the motor from the 

 anterior side of the cord. Soon after leaving 

 the openings between the vertebrae these 

 nerves unite and form a small network or 

 ganglion, as shown in the illustration. 



Physiology oj the Cerebrospinal System. The 

 nerves of the cerebrospinal system work in 

 pairs, the sensory nerves conveying impulses to 

 the brain and the motor nerves conveying im- 

 pulses from the brain. You say, "John, please 

 close the door." Your words are carried to the 

 auditory centers of the brain through the nerves 

 of hearing, and the brain sends to the necessary 

 muscles the impulse to perform the act. You 

 hand me a book and ask me to read to you. 

 The forms of the words on the printed page 

 are conveyed to the brain through the nerves 

 of sight, and the brain sends impulses through 

 certain motor nerves that cause me to utter the 

 words aloud. You can recall hundreds of illus- 

 trations of this sort of nerve action. 



Sometimes, as in case of injury or fright, the 

 motor impulse is sent over the nerves before 

 the sensory impulse has reached the brain, the 

 communication of one set of nerves with the 



