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HUMAN PHYSIOLOGY 



A nerve cell from the brain of a mon- 

 key, showing a great number of 

 dendrites. 



serve as pathways between the end organs of touch, sight, taste, 

 etc., and the centers in the brain or spinal cord. Thus sensation 

 is obtained. 



Nerve Cells and Fibers. — A nerve cell, like most other cells in the body, 

 is a mass of protoplasm containing a nucleus. The body of the nerve cell is 



usually rather irregular in shape, and dis- 

 tinguished from most other cells by pos- 

 sessing several delicate, branched proto- 

 plasmic projections called dendrites. 

 (See figure.) One of these processes, the 

 axis cylinder process, is much longer than 

 the others and ends in a muscle or organ 

 of sensation. The axis cylinder process 

 forms the pathway over which nervous 

 impulses travel to and from the nerve 

 centers. 



Nerves consist of bundles of such tiny 

 axis C3dinder processes, bound together by 

 connective tissue. As a nerve ganglia is 

 a center of activity in the nervous sys- 

 tem, so a nerve cell is a center of activity 

 which may send an impulse over this thin strand of protoplasm, the axis 

 cjdinder process, prolonged into a nerve fiber many hundreds of thousands 

 of times the length of the cell. Some nerve cells in the human body, al- 

 though visible only under the compound microscope, give rise to axis cylin- 

 der processes several feet in length. Because some nerve fibers originate 

 in organs that receive sensations and send those sensations to the central 

 nervous system, they are called sensory nerves. Other axis cylinder pro- 

 cesses originate in the central nervous system and pass outward as nerve 

 fibers; such nerves produce movement of muscles and are called motor nerves. 



The Cerebrospinal Nervous System of the Frog. — For this exercise use frogs 

 that have been preserved in alcohol. Those previously used for other work 

 on the anatomy of the frog will do perfectly well. 



With a sharp knife or scalpel, cut away the skin from the top of the head 

 and along the back. Cut carefully through the top of the cartilaginous 

 skull. The brain will then be exposed, lying in a bony cavity surrounded 

 by a watery fluid, the function of which is to protect the delicate brain from 

 shock. Notice the white elongated hemispheres of the forehrain or cerebrum. 

 The two anterior projections of the cerebrum are called the olfactory lobes. 

 It is by means of the olfactory lobes that odors are perceived. Posterior 

 to the cerebrum and connected with it is the midbrain. The dorsal side is 

 enlarged to form a pair of optic lobes. How do the optic lobes compare in 

 size with the cerebrum ? Insert the blunt end of a scalpel, and turn the optic 

 lobes slightly so as to find the optic nerves. Notice that they cross each 

 other, the one from the right optic lobe going to the left eyeball, the one 



