282 STRUCTURE OF THE VERTEBRATES 



laterally, between the dorsal and ventral roots. In this same 

 region, but more central in position, are a number of motor 

 tracts. The ventral segment of the cord is almost entirely motor. 



A destruction of the motor tracts causes a loss of voluntary 

 action, for the connections between the brain and the motor 

 nerves of the cord would then be lost. The reflex pathways would 

 not be affected; but reflex movements w^ould be exaggerated for 

 the coordinating influence of the brain would be lost. If, how- 

 ever, the motor cells of the cord are destroyed, both voluntary 

 and involuntary reactions would cease in the regions supplied 

 by affected nerves. Motor impulses from the brain would con- 

 tinue to pass down the cord but could not be relayed to the 

 effectors. As a result of any such destruction of cells the mus- 

 cles, including those of the arteries, lose their power to react; 

 and a degenerative condition follows. Infantile paralysis is 

 caused by the destruction of ventral motor nerve cells. 



Thus far the fibers of the cord and spinal nerves have been 

 classified simply as motor or sensory ; but each of these types can 

 be divided into somatic fibers which pass to or from the muscles 

 and skin, and visceral fibers to or from the internal organs. 

 Each spinal nerve therefore contains four types of fibers: vis- 

 ceral and somatic motor, and visceral and somatic sensory. The 

 visceral fibers form the connection between the central nervous 

 system and the sympathetic ganglia. 



D. Sympathetic System 



In addition to the central nervous system and the cranial and 

 spinal nerves which arise from it, there are a number of vis- 

 ceral ganglia which are not affected by voluntary motor im- 

 pulses. The nomenclature of this vegetative nervous system has 

 never been standardized. The simpler terminology uses autonom- 

 ic to describe this entire system of ganglia and nerves which 

 are involuntary in action. The ganglia of the thoracic region 

 form the sympathetic part of the system. Those ganglia which 

 lie in the cervical and lumbar regions are the parasympathetic. 

 The classification is based upon position and the physiological 

 reactions of the ganglia. 



Lying immediately ventral to the centra of the vertebrae, and 



