THE FROG 77 



Add the spinal nerves to your drawing of the central 

 nervous system; represent them twice natural size. 



3. The sympathetic system centers in two delicate chains 

 of ganglia extending longitudinally, one on each side of the 

 body, lying close to the dorsal aorta and systemic arches. 

 Turn the viscera to one side and lift up the kidney; this will 

 expose the sympathetic system of one side. A nerve trunk 

 unites the ganglia of each chain. About how many ganglia 

 are there? Are the sympathetic ganglia united with the 

 spinal nerves ? If so, how? The sympathetic system inner- 

 vates the internal organs generally. Branches of the sym- 

 pathetic unite to form plexuses, the largest of which, the 

 solar plexus, sends branches to the stomach and adjacent 



organs. 



Add the main trunks of the sympathetic system to your 

 drawing of the central nervous system; show also the con- 

 nections between the sympathetic system and the spinal 

 nerves. 



B. The Finer Structure of the Nervous System.- 

 Nervous tissue is made up of two different elements : (a) 

 the nervous elements proper, called nerve cells, ganglion 

 cells or neurons, and (&) neuroglia cells and connective 

 tissue, whose function is to support and bind the nerve cells 

 together. 



A nerve cell or neuron consists of a compact or angular 

 cell body, containing the nucleus, and numerous slender 

 processes of which one or more may be very long and are 

 distinguished as axons, or nerve fibers. The essential part 

 of an axon or nerve fiber is the axis cylinder, and in some 

 cases this is the only part present; but usually the axis 

 cylinder is enclosed in two additional layers, described be- 



