CRANIAL NERVES. 91 



Law of Contraction. If a feeble galvanic current be applied to a 

 recent and excitable nerve, contraction is produced in the muscles only 

 upon the making of the circuit with both the direct and inverse currents. 



If the current be moderate in intensity, the contraction is produced in 

 the muscle both upon the making and breaking of the circuit, with both 

 the direct and inverse currents. 



If the current be intense, contraction is produced only when the circuit 

 is made with the direct current, and only when it is broken with the inverse 

 current. 



The Reaction of Degeneration. Two different applications of elec- 

 tricity are used in electro-physiology and electro-therapeutics the constant 

 or galvanic, and the interrupted or faradic currents. Injured and paralyzed 

 muscles and nerves react differently to these two kinds of stimuli, and 

 the facts are of the greatest importance in the diagnosis and therapeutics 

 of the precedent lesions. The principal difference of behavior relates to 

 the reaction of degeneration a condition produced by paralysis of any 

 kind. It is characterized by a diminished or abolished excitability of the 

 muscles to the faradic current, while there is at the same time an increased 

 excitability to the galvanic current. The synchronous diminished excita- 

 bility of the nerves is the same for either current. The term partial reac- 

 tion of degeneration is used when there is a normal reaction of the nerves, 

 but the muscles show the degenerative reaction. This condition is a 

 characteristic of progressive muscular atrophy. 



CRANIAL NERVES. 



The Cranial Nerves come off from the base of the brain, pass through 

 the foramina in the walls of the cranium, and are distributed to the skin, 

 muscles and organs of sense in the face and head. 



According to the classification of Soemmering, there are 12 pairs of 

 nerves, enumerating them from before backward, as follows, viz. : 

 ist Pair, or Olfactory. 7th Pair, or Facial, Portio dura. 



2d Pair, or Optic. 8th Pair, or Auditory, Portio mollis. 



3d Pair, or Motor oculi communis. Qth Pair, or Glosso-pharyngeal. 

 4th Pair, or Patheticus, Trochlearis. loth Pair, or Pneumogastric. 

 5th Pair, or Trifacial, Trigeminus. nth Pair, or Spinal accessory. 

 6th Pair, or Abducens. I2th Pair, or Hypoglossal. 



The Cranial Nerves may also be classified physiologically, according 

 to their function, into three groups : I . Nerves of special sense. 2. Nerves 

 of motion. 3. Nerves of general sensibility. 



