THE FROG AND VERTEBRATES IN GENERAL 215 



of fatty tissue, the medullary sheath. Each axon in this cable- 

 like nerve is thus separated, or as we might say, insulated, from 

 all the other nerve elements by the medullary sheath and neu- 

 rilemma, and all the elements in a nerve are bound together by 

 the connective tissue fibers which are continuous with the outer 

 perineurium. A nerve fiber, such as just described, is known as 

 the medullated type in distinction from the non-medullated 

 type, frequently found in the autonomic nervous system, in which 

 the medullary sheath is lacking. (W. f. 138, D.) 



III. AUTONOMIC NERVOUS SYSTEM 



There are two main nerve trunks in this division of the nervous 

 system which lie dorsally in the coelom, one on either side of the 

 spinal column. Each trunk originates in the head region in a 

 prootic ganglion which, as stated, is formed by the fifth, sixth, and 

 seventh cranial nerves, and may be regarded as a chain of ganglia. 

 The ganglia receive one or more branches (ramus communicans) 

 from each of the spinal nerves shortly after it emerges between 

 the vertebrae. (W. f. 142, Sg. 1, 10.) 



Branches from the autonomic system innervate various impor- 

 tant organs of the body. In the anterior part of the body these 

 branches innervate the muscular walls of the subclavian and occip- 

 ito- vertebral arteries, as well as the anterior ends of the oviducts. 

 Farther posteriorly, a number of branches form a very important 

 nerve center, the solar plexus, from which nerves are distributed 

 to the important abdominal organs including the stomach, in- 

 testine, liver, pancreas, and parts of the urogenital organs. Still 

 farther posteriorly, branches from this system form the uro- 

 genital plexus which innervates various urogenital organs. The 

 autonomic nervous system is responsible for the involuntary con- 

 trol of many of the most important organs of the body, acting 

 largely through the medium of the unstriated muscle tissue. 



iv. sense organs 



The sense organs are composed of essential and accessory 

 parts. The essential part of a sense organ consists of peripheral 

 sensory cells which are capable of receiving one or more types of 

 stimuli from the external environment. Accessory structures are 

 also generally present in sense organs, which aid in various ways 

 in bringing the stimulus to the nerve tissue. For example, the 



