The Frog. S [ 



fully, and possibly even functionally, developed as an eye in 

 certain other vertebrates. Behind the thalamencephalon come- 

 the two optic lobes, the corpora bigemina, as they are sometimes 

 termed. Then a narrow band of brain tissue stretches across 

 to form the cerebellum, rudimentary in the frog, but of great 

 importance in the higher vertebrates. Behind this, again, is the 

 medulla oblongata, which gradually narrows into the spinal cord. 

 Ten pairs of nerves arise from the brain, which are as follows : 

 (i) olfactory, supplying nose; (2) optic, supplying eyes; (3) 

 motores oculorum, supplying most of the muscles of the eye ; 

 (4) pathetici, supplying the superior oblique muscles of the 

 eye ; (5) trigeminus, with three branches running to the skin of 

 the front part of the head and the lower jaw, ; (6) abducentes, 

 supplying the external rectus and retractor bulbi muscles of 

 the eye ; (7) thefaciales, supplying the roof and the floor of the 

 rhouth ; {8) the auditory nerves, going to the ear ; (9) the 

 glossopharyngei, to the root of the tongue ; (10) the vagi, supply- 

 ing the dorsal integument of the head and trunk, and the heart, 

 lungs, and stomach. 



The spinal cord gives off ten pairs of nerves ; the first is the 

 hypoglossal, which supplies certain muscles at the back of the 

 head ; the next two nerves unite a short way from their origin, 

 and form a trunk supplying the fore limb. This union between 

 the two nerves is known as the brachial plexus . The seventh 

 to the tenth spinal nerves form another plexus, which is con- 

 cerned with the nerve-supply of the hind Kmb. 



The sympathetic system consists of a chain of ganglia on 

 either side of the aorta. The renal organs of the frog consist 

 of a pair of kidneys, which really do not deserve the name of 

 kidneys, as they correspond to the mesonephros of the embryo 

 fowl (see p. 140). They are reddish bodies, and on the surface 

 of each is a yellowish band, the adrenal body. The duct of 

 each mesonephros opens into the cloaca, that of the male 

 being provided with a little glandular caecum known as the 

 vesic%da seminalis. From the cloaca in both sexes arises a 

 bilobed bladder. In the male frog there are a pair of egg- 

 shaped testes ; the ducts from these pass through the meso- 

 nephros, and reach the exterior by the mesonephric ducts (the 



G 



