THK SPUSTAL CORD OF THE FROG 389 



backward to the beginning of the spinal cord, is called the 

 hindbrain. It shows two regions. The transverse ridge 

 just back of the optic lobes is called the cerebellum, while 

 the wedge-shaped portion baick of this, from which the 

 spinal cord arises, is called the medulla oblongata. Seven 

 pairs of cranial nerves arise from this medulla. The fourth 

 pair, known as the trochlears, are very small, and carry mo- 

 tor impulses to one of the muscles which move the eyeballs. 

 The fifth pair, or trigeminals, are very large. They belong 

 to the class of mixed nerves. Some of their fibers bring 

 sensory impulses from the teeth and face to the brain. 

 Other fibers carry motor impulses to the muscles of the 

 jaws and eyelids. The sixth pair, or abducents, carry mo- 

 tor impulses to a muscle of the eyeball. The seventh pair, 

 like the fifth, are facial nerves of a mixed character, and 

 carry impulses to and from certain parts of the face. In 

 the higher animals and in man this nerve is purely a motor 

 nerve, and controls the muscles which give expression to 

 the face. The eighth pair, or auditory nerves, are sensory, 

 and carry impulses of hearing from the ears. The ninth 

 pair, or glossopharyngeal, supply the tongue and pharjoix, 

 and are mixed nerves. The tenth pair, or the vagus nerves, 

 have the widest distribution of any. They are also mixed 

 nerves, and their fibers supply the lungs and air passages, 

 the heart and blood vessels, and the organs of digestion. 



Finally, the centers of all these cranial nerves are in 

 communication with parts of the forebrain, and the con- 

 sciousness of sensations has its seat in that organ, though 

 the nerves are not directly attached to it. 



The spinal cord of the frog. — The cord of the frog, which 

 is merely an extension of the medulla, is much shorter 

 than that of man. From it are given off some ten pairs 



