THE FROG 67 



form. From various experiments, however, it is known that the frog 

 loses the power of spontaneous movement if the mid brain and cerebral 

 hemispheres are removed, while the spinal cord becomes very irritable 

 if the optic lobes are cut away. No function has yet been definitely 

 ascribed to the cerebellum and even when all of the brain, with the ex- 

 ception of the medulla, is removed, the animal "breathes normally, snaps 

 at and swallows food, leaps and swims regularly, and is able to right 

 itself when thrown on its back." If the posterior portion of the medulla 

 is removed, the frog dies. 



THE SPINAL CORD 



The spinal cord passes down through the bony vertebral or spinal 

 column. It is short and somewhat flattened. There is an enlargement 

 in the brachial region where the nerves pass off to the fore limbs, and 

 one further back where the large nerves originate which supply the hind 

 legs. The cord tapers to a narrow thread called the filum terminate 

 which extends into the urostyle. There is a median fissure on both dor- 

 sal and ventral sides of the cord, while from the sides of it, the roots 

 of the spinal nerves are given off. The cord itself is surrounded by two 

 membranes, an outer, the dura mater, and an inner known as the pia 

 mater. There is an H-shaped central mass of gray matter consisting- of 

 nerve cells, and an outer mass of white matter composed of nerve fibers. 



There is a little opening through the center of the cord called the 

 central canal. The various cavities in the brain are a continuation and 

 expansion of this central canal. 



THE PERIPHERAL NERVOUS SYSTEM 



There are ten pairs of spinal nerves in the frog, each arising by a 

 dorsal and ventral root and springing from the horns of the gray matter 

 of the cord (Fig. 470). The two roots unite to form a trunk, passing 

 out between the arches of the vertebrae. 



The brachial, or arm branches, are made up of the second, as well as 

 branches from the first and third pairs of spinal nerves, and pass to the 

 fore limbs and shoulder, while the sciatics arise from plexuses composed 

 of the seventh, eighth, and ninth spinal nerves, and run to the legs. 



There are also ten pairs of cranial nerves which supply the organs 

 of special sense, certain muscles, various organs of the head, the heart, 

 lungs, and stomach. They are named as follows :* 



*There are two additional cranial nerves in the higher animals, the spinal accessory and hypo- 

 glossal, and medical students remember them by the following verse, the first letter of each word 

 being the initial letter of the correspondingly numbered nerve: 



I. On VII. Finn 



II. Old VIII. And 



III. Olympus IX. German 



IV. Towering X. Picked 

 V. Tops XT. Some 



VI. A XII. Hops 



