THE FROG — A REPRESENTATIVE VERTEBRATE 417 



medulla also has a thin roof which forms a choroid plexus dipping 

 into the large fourth ventricle. The choroid plexuses secrete a cerebro- 

 spinal fluid which fills the ventricles and central canal. Some of this 

 fluid escapes via pores in the roof of the medulla to circulate between 

 the brain and cord and certain of their meninges, or connective tissue 

 sheaths. It forms a protective liquid cushion and helps nourish the 

 central nervous tissue. 



Ten pairs of cranial nerves extend from the brain to various parts 

 of the body. The first pair are the olfactory nerves (I), which bring 

 impulses from the olfactory epithelium to the olfactory bulbs and 

 cerebral hemispheres. Fibers in the optic nerves (11) come from the 

 retina, cross to the opposite side of the brain, forming an optic chiasma 

 on the ventral surface of the diencephalon, then continue as optic 

 tracts to end chiefly in the optic lobes. The oculomotor (HI), trochlear 

 (IV) and abducens (VI) nerves contain motor fibers to the muscles that 

 move the eyeball. The third also includes motor fibers to muscles 

 within the eye that move the lens. The trigeminal nerve (V) brings in 

 sensory impulses from the skin of the head, and carries motor impulses 

 to the jaw muscles. The facial nerve (VII) is also mixed, supplying 

 motor fibers to certain of the throat muscles and to the tear glands, and 

 sensory fibers to the mouth and pharynx. Many of the latter innervate 

 taste buds. The acoustic nerve (VIII) brings impulses from the inner 

 ear to the anterior portion of the medulla. The glossopharyngeal 

 nerve (IX), like the facial, conducts sensory impulses from the mouth 

 and pharynx, and carries motor impulses to a few throat muscles. The 

 last of the frog's cranial nerves, the vagus (X), is attached to the side 

 of the medulla in common with the glossopharyngeal nerve. It supplies 

 motor and sensory fibers to the posterior part of the pharynx, certain 

 of the shoulder muscles and most of the abdominal viscera (heart, lungs, 

 digestive tract). 



Like the entire trunk region of the frog, the spinal cord is short, 

 and the number of spinal nerves is reduced to ten pairs. Each of the 

 spinal nerves is attached to the cord by a dorsal and a ventral root 

 (Fig. 29.10). The former contains sensory fibers and an enlargement, the 

 dorsal root ganglion, in which the cell bodies of these neurons are 

 located; the latter, motor fibers. The roots join peripherally and the 

 spinal nerves are mixed. As the spinal nerves emerge from the vertebral 

 column, they are surrounded by calcareous bodies of uncertain signifi- 

 cance. They are then distributed to the trunk and limbs in the manner 

 illustrated in Figure 21.19. The first spinal nerve supplies the tongue 

 muscles. This nerve is actually comparable to the second spinal nerve 

 of other vertebrates, for a more anterior spinal nerve is lost during 

 embryonic development. Each spinal nerve has a ventral branch, the 

 ramus communicans, which passes to a ganglionic enlargement on the 

 sympathetic cord-a pair of longitudinal nerve tracts lying on each side 

 of the dorsal aorta. A pair of splanchnic nerves extends from the 

 sympathetic cords along the coeliacomesenteric artery to the abdominal 

 viscera. The motor fibers in the sympathetic cords and splanchnic 



