596 VeRTEBRATE LIFE AND ORGANIZATION 



nerves in the sacral region. Preganglionic: parasympathetic fibers are 

 longer than those of the sympathetic system tor they end in ganglia that 

 are very near the organs they supply, or are in the walls oi the organs. 

 Relatively short postganglionic parasympathetic fibers continue to the 

 muscle and gland cells. 



Sympathetic and parasympathetic systems have opposite effects upon 

 the organs innervated. Sympathetic stimulation speeds up the rate and 

 increases the force of the heart beat, causes arteries to constrict, thereby 

 increasing the blood pressure, increases the glucose content of the blood, 

 and in general has effects that enable the body to adjust to conditions 

 of stress. It inhibits the secretion of the salivary glands and the activity 

 of the digestive tract generally. Parasympathetic stimulation, on the 

 other hand, speeds up salivary secretion, peristalsis of the digestive tract 

 and similar vegetative processes, but it slows down the heart and de- 

 creases blood pressure. 



Ingenious experiments performed by Loewi in I92I demonstrated 

 the cause of the opposite effects of sympathetic and parasympathetic 

 fibers. He removed the heart of a frog, leaving only its nerve supply 

 intact, then perfused a salt solution through it and into another com- 

 pletely isolated heart. Both hearts continued to beat. When the vagus 

 nerve (parasympathetic fibers) going to the first heart was stimulated, 

 the rate of both hearts slowed down; when the sympathetic fibers were 

 stimulated, the rate of both hearts increased. Apparently some substance 

 secreted by the nerves going to the first heart entered the salt solution and 

 reached the second heart. Further work revealed that two neurohumors 

 are produced. Acetylcholine is secreted by the parasympathetic and 

 sympathin by the sympathetic fibers. Acetylcholine may also be involved 

 in the transmission of the nerve impulse across the synapses in other parts 

 of the nervous system, and across the junction between neuron and 

 muscle. It may also play a role in the transmission of the nerve impulse 

 along the neuron. Sympathin has been found only in connection with 

 postganglionic sympathetic fibers, but it is closely related to epinephrine, 

 secreted by the mecluUary cells of the adrenal gland. There is fairly clear 

 evidence that these cells are themselves modified postganglionic sym- 

 pathetic fibers. 



247. Central Nervous System 



Spinal Cord. A small central canal (Fig. 29.10) extends through 

 the center of the spinal cord, gray matter surrounds the central canal, 

 and white matter lies peripheral to the gray. The gray matter is dark in 

 color, for it is composed of the cell bodies of neurons and of unmy- 

 elinated fibers; the white matter is light, because it is composed of fibers 

 surrounded by fatty myelin sheaths. The gray matter forms continuous 

 longitudinal columns, which are H-shaped in cross section. There is a 

 pair of dorsal columns, a pair of ventral columns, and a gray commis- 

 sure connecting the columns of opposite sides. The dorsal column con- 

 tains the dendrites and cell bodies of afferent internuncial neurons, with 

 which many afferent neurons synapse. The ventral column contains the 



