THE FROG, A TYPICAL VERTEBRATE 259 



The brain is made up of three primary vesicles, a fore-brain, 

 mid-brain, and hind-brain. The fore-brain gives rise to a pair 

 of cerebral hemispheres (Fig. 147, 1), the mid-brain to a pair of 

 optic lobes (3), and the hind-brain to the cerebellum (4) and 

 medulla oblongata (5). It is not certain what the functions of 

 the cerebral hemispheres are in the frog; they are the seat of 

 intelligence and voluntary control in higher animals. The 

 brain as a whole controls the actions produced by the nerve 

 centers of the spinal cord. " The higher centers of the brain 

 are comparable to the captain of a steamer who issues orders to 

 the man running the engine when to start and when to stop, 

 and who has his hand on the wheel so as to guide the course of 

 the vessel." 



The ten pairs of nerves that arise from the brain of the frog 

 are known as cranial nerves. Some of these are sensory, others 

 motor in function. They are distributed to the nose (Fig. 147, 

 o.n), eye, inner ear, skin and muscles of the face, muscles of the 

 jaws, tongue, and pharynx, and to the throat, lungs, heart, 

 stomach, and intestine. Many of the vertebrates possess two 

 more pairs of cranial nerves than the frog. 



The spinal cord is a thick tube directly connected with the 

 brain (Fig. 147, s.c); it passes through the neural arches of the 

 vertebral column. It is composed of a central mass of gray 

 matter (Fig. 148, g.m), consisting mainly of nerve cells, and an 

 outer mass of white matter (w.m) made up chiefly of nerve fibers. 



The relation of the spinal nerves to the spinal cord and the 

 paths taken by nervchis impulses are indicated in Figure 148. 

 There are ten pairs of spinal nerves in the frog. Each arises by 

 a dorsal root (Fig. 148, d.r) and a ventral root (v.r) which spring 

 from the horns of the gray matter of the cord. The two roots 

 unite to form a trunk, which passes out between the arches of 

 adjacent vertebrae. 



The functions of nervous tissue are perception, conduction, 

 and stimulation. These are usually performed by nerve cells, 

 called neurons. The reflex is considered the physiological unit 



