Chap. 34 AMPHIBIANS 707 



body. Frogs with all of the brain removed except the cerebellum and medulla 

 can still move about more or less normally, will croak when properly stimu- 

 lated and can breathe regularly. 



METENCEPHALON. This vcry short section is here roofed by the narrow 

 cerebellum; it is relatively large in higher vertebrates. Experiments show that 

 it is a center of muscular coordination. 



MYELENCEPHALON. The sides and floor of the myelencephalon make up the 

 medulla oblongata which is composed of nerve cell processes extending to and 

 from the spinal cord and parts of the brain. Processes of its cell bodies extend 

 to the autonomic nervous system (parasympathetic) that controls breathing 

 movements and the action of the heart. It contains the fourth ventricle which 

 tapers posteriorly into the central canal of the cord. The former is covered by 

 the posterior choroid plexus, and in freshly killed frogs it is colored red by its 

 abundant capillaries. 



After all of the brain except the medulla has been removed, a frog is in- 

 active apparently with comfort. It will swallow food placed well down its 

 throat and, properly cared for, may live for some time. Removal of the whole 

 medulla kills the animal since this region controls the breathing movements, 

 contraction of the walls of the blood vessels and the action of the heart. 



Spinal and Cranial Nerves. The spinal and cranial nerves are the roadways 

 over which pass all the countless messages of the frog's awareness of and ad- 

 justment to its surroundings. Ten pairs of spinal nerves branch from the sides 

 of the cord and extend out through openings between the vertebrae (Fig. 

 34.21 ). Each nerve has two roots. The dorsal or sensory root contains nerve 

 cell processes (afferent) over which nerve impulses from sensory cells such as 

 touch pass into and up the cord. The bodies of the cells over which the im- 

 pulses go are grouped together in a ganglion on the dorsal root. These ganglia 

 are covered by white chalky deposits, the calciferous bodies, pouches of the 

 dura mater filled with granules of calcium carbonate. The ventral or motor 

 root of the same nerve contains processes over which impulses, initiated in the 

 brain or cord, pass from cells in the cord to the muscles and direct their move- 

 ment. Processes of sensory cells and motor cells lie side by side in the same 

 spinal nerve, but impulses from the skin always come in on the sensory ones 

 and impulses from the cord to the muscle always go out over the motor ones. 

 It is a strictly one way system, like messages passing one another on different 

 telephone wires. Impulses go over the complete sensory-motor circuit when 

 something touches a frog's foot and it moves away. 



Ten pairs of cranial nerves branch from the brain of the frog. Some are 

 sensory, like the olfactory nerves, others are motor such as the oculomotors 

 through which the movements of the eyeballs are controlled (Figs. 16.13, 

 34.22). Most of the cranial nerves have single roots and do not occur at 

 such regular intervals as the spinal nerves. 



