284 THE BIOLOGY OF THE FROG chap. 



part. The nerves correspond to the wires, and the ganglia 

 to the central stations where messages may be transferred 

 from one line to another. All parts of the body are sup- 

 plied with nerves which are connected with the central 

 nervous system, and through this channel connections may 

 be established between any two or more parts of the organ- 

 ism. In this way there is rendered possible the coordina- 

 tion of movements in different parts of the body, and the 

 ability of the organism to act as a whole in relation to 

 external things. 



The nervous system is composed of three rather closely 

 associated divisions : the cerebrospinal, consisting of the 

 spinal cord and brain ; the peripheral, consisting of the 

 spinal and cranial nerves ; and the sympathetic. 



The Spinal Cord. — The spinal cord of the frog is short 

 and somewhat flattened. It presents two enlargements, one 

 in the brachial region, where the large nerves to the fore 

 limbs are given off, and one farther back, where the large 

 nerves originate which supply the hind legs. Behind the 

 posterior enlargement the cord tapers to a narrow thread, 

 the filum terminate, which extends into the urostyle. At 

 its anterior end the cord widens gradually into the medulla 

 oblongata, the posterior division of the brain. Both the 

 dorsal and the ventral sides of the cord are divided by a 

 median fissure. At the sides of the cord the roots of the 

 spinal nerves are given off ; each nerve arises from a dorsal 

 and a ventral root which combine just after they emerge 

 from the vertebral canal through the intervertebral foramina. 

 The roots of the posterior spinal nerves are much elongated, 

 inasmuch as the shortening of the cord brings their origin 

 far in front of the vertebrae to which they correspond ; the 

 bundle of roots thus formed, together with the filum termi- 

 nate, is known as the cauda equina. 



