ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 417 

 GENERAL DISTRIBUTION OF THE SPINAL NERVES. 



It would be quite undesirable from the standpoint of an 

 elementary text-book to give with surgical exactness the 

 distribution of these thirty-one pairs of spinal nerves. 

 Suffice the general statement that these spinal nerves sup- 

 ply the voluntary muscles of the neck and the trunk ; that 

 as the phrenic nerve they control the diaphragm; as sen- 

 sory nerves they are distributed to the entire skin of neck, 

 trunk and limbs, while as motor nerves they innervate the 

 muscles of the limbs and figure in all their voluntary move- 

 ments. In a word, it may be said that the spinal nerves 

 innervate all that portion of the body below the head from 

 which we derive special sensations or in which we are able 

 to produce voluntary movements. In addition to this, com- 

 municating branches reach the sympathetic system and 

 bring it into physiological connection with the brain and 

 spinal cord. 



THE BRAIN. 



Under the term "brain" is included all that portion of 

 the cerebro-spinal system lying above and including the 

 medulla oblongata. It consists of three main divisions ap- 

 parent at once to the unaided eye: the large fore-brain or 

 cerebrum, the hind-brain consisting of the cerebellum and 

 the medulla oblongata lying immediately below it, and the 

 mid-brain lying between the cerebrum and hind-brain and 

 consisting of the corpora quadrigemina and crura cerebri. 



Weight. The weight of the brain varies considerably, 

 an average weight being in the neighborhood of fifty 

 ounces in the adult male and about forty-five in the female. 

 Of this weight of fifty ounces the fore-brain or cerebrum 

 weighs about forty-four, being therefore very much larger 

 than the hind-brain and mid-brain together. In fact, it 

 laps entirely over the other portions, so that a view from 

 above would not disclose either the cerebellum or mid- 

 brain. This especial development of the fore-brain is, how- 

 ever, a characteristic of the human species alone. As we 



