The Frog 67 



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 

 exception 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 

 dorsal and ventral sides, while from the sides of the cord 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 



The frog has ten pairs of spinal nerves, 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 be- 

 tween 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 i 1 



1. The olfactory ( ) nerves, running from the 



olfactory lobes to the nasal cavities. 



1 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 



II. Old 



III. Olympus 



IV. Towering 

 V. Tops 



VI. A 



VII. 



Finn 



VIII. 



And 



IX. 



German 



x. 



Picked 



XI. 



Some 



XII. 



Hops 



