THE SPECIAL ANATOMY OF THE FROG 49 



hind limbs. The small tenth nerve arises from the urostyle, and innervates 

 mainly the cloaca, and urinary bladder. 



Draw the spinal nerves as seen on the dorsal body wall. 



7. The roots of the spinal nerves. Select one of the largest spinal nerves 

 (as the eighth), and trace it carefully into the vertebral column, cutting away the 

 vertebrae. Pull off the calcareous body and find within it a small brown object, 

 the dorsal or spinal ganglion. (The term "ganglion" means a mass of nerve 

 cell bodies lying outside of the brain and spinal cord.) Tracing the nerve farther 

 in toward the cord note that it divides into two branches or roots, one of which 

 is attached to the dorso-lateral region of the cord, the other to the ventro- 

 lateral region. These roots are designated as the dorsal and ventral roots. The 

 dorsal root springs from the dorsal horn of the gray matter of the cord; it carries 

 sensory fibers which arise from the nerve cells located in the dorsal ganglion. 

 The ventral root takes its origin from the ventral horn of the gray matter, from 

 the large motor cells which have already been seen in that location, and carries 

 motor fibers to the muscles. The two roots meet just beyond the spinal ganglion, 

 which is on the dorsal root, and the spinal nerve thus formed soon divides into 

 dorsal and ventral branches or rami, as noted in the preceding section. Each 

 ramus carries both sensory and motor fibers and supplies both skin and muscles; 

 and the ventral ramus is further connected by the ramus communicans with the 

 sympathetic system. 



Make a diagram to show the origin of a spinal nerve from the cord. 



8. General remarks on the function of the brain and cord. The possibility 

 that any organism born into the conditions of life as they exist upon the earth 

 will survive depends entirely upon its ability to perceive and respond effectively 

 to those conditions. We have already found that this capacity for the percep- 

 tion of conditions in the environment and for responding to them is vested in the 

 nervous system. The perceiving part of the apparatus is the sense organs, 

 which comprise the eye, ear, and nose, taste organs in the mouth, organs of 

 touch, pressure, pain, temperature, chemical sense, etc., in the skin, and sensory 

 organs in the viscera. The responding part of the apparatus is the brain and 

 cord through the motor nerves to muscles and glands. 



In a prone animal moving with one end forward, that end will come first in 

 contact with the factors of the environment and hence will naturally- come to 

 be the place where the most important and specialized sense organs are located. 

 Further, the part of the nervous system connected with these important sense 

 organs must become enlarged to accommodate the numerous impulses sent in 

 from them, and must thus acquire dominance over the rest of the central nervous 

 system. We may thus account for the origin of the head and brain, which 

 structures indeed made their appearance in the simplest bilateral animals. 



We therefore find that the brain of a relatively simple vertebrate like the 

 frog consists in large part of centers for the reception of the chief sensations. 



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