The Frog — An Amphibious Vertebrate Animal 437 



The spinal cord has a dorsal median fissure and a ventral median fis- 

 sure. The cord is composed of a central mass of gray matter (principally 

 nerve cells) in the shape of the letter H and an outer mass of white mat- 

 ter made up of nerve fibers. The hollow central canal extends through- 

 out the entire cord and may be seen in the middle of the crossbar of the 

 H in a cross-section of the spinal cord (Fig. 17). The central canal con- 

 nects anteriorly with the cavities (ventricles) of the brain. The spinal 

 cord has two surrounding membranous meninges, the outer one being 

 called the dura mater and the inner, the pia mater. 



There are ten pairs of spinal nerves, each arising from the gray matter 

 of the spinal cord by a dorsal root and a ventral root (Fig. 17). The 

 union of these two roots at the side of the cord forms a spinal nerve. 

 Each spinal nerve passes out between the bony arches of adjacent 

 vertebrae. 



OLFACTORY LOBE 



OPHTHALNAIC 



PALATINE 



;v\AX\LLARY 

 MANDIBULAR 



FACIAL 



OLFACTORY 



EYE 



EAR 



CLOSSOPHARYNGEAL- 

 CEREBELLUM 



VAGUS 



FOURTH VENTRICLE 



RRST SPINAL- 



CEREBRUM 

 GASSERIAN 

 GANGLION 



OPTIC LOBE 



JUGAL GANGLION 



MEDULLA 



SYMPATHETIC 

 NERVES 



SPINAL CORD 



Fig. 217. — Brain and cranial nerves of the bullfrog {Rana catesbeiana) shown 

 somewhat diagrammatically from the dorsai side. 1 to 10 show the cranial nerves; 

 11, the first pair of spinal nerves. Certain cranial nerves show some of their 

 branches. (From Atwood: A Concise Comparative Anatomy, The C. V. Mosby 

 Co.) 



The sympathetic nervous system consists of two main trunks which 

 parallel the spinal cord, one on either side of it. Each trunk has ten 

 ganglia or enlargements where the ten pairs of spinal nerves unite with it. 



The skin, because of its contained sensory nerve endings, receives 

 tactile, chemical, heat, and light stimuli (Fig. 208). The paired eyes 

 have a large, spherical lens which, in other respects, resembles the eyes 

 of other vertebrates. There are three eyelids: the rather motionless 



