SENSE ORGANS AND NERVOUS COORDINATION 



591 



neuron continues to be stimulated. When the stimulation stops, the 

 passage of impulses stops. There are other arrangements in the nervous 

 system that ensure the continuation of the impulse for a period of time 

 after the stimulus has stopped. One of these is tlie multiple chain circuit 

 (Fig. 29.9, C). The first neuron is stimulated momentarily, an impulse 

 travels rapidly to the terminal neuron, and also via a branch to a second 

 neuron. The second neuron is stimidated and a moment later sends a 

 second impulse to the same terminal neuron and also via a branch to a 

 third neuron, which is stimulated and sends yet a third impulse to the 

 same terminal neuron. If a great many neurons are involved, the ter- 

 minal neuron will receive a whole series of impulses, and receive them 

 for some time after the initial stimulus has stopped. In another arrange- 

 ment, the closed chain circuit (Fig. 29.9, D), one or more branches of the 

 neurons in the circuit feed back to a point near the beginning of the 

 circuit. Once such a circuit is activated, impulses could continue in- 

 definitely unless the neurons became fatigued or were inhibited. Presum- 

 ably such circuits form the basis for the spontaneous activity of the 

 inspiratory center and similar centers in the brain. 



246. Peripheral Nervous System 



Spinal Nerves. The vertebrate body is segmented (although seg- 

 mentation is obscure in the head region) and there is a pair of peripheral 

 nerves for each body segment: those arising from the spinal cord are 

 known as spinal nerves; those from the brain, as cranial nerves. Afferent 

 and efferent neurons lie together in most of a spinal nerve, but near 

 the cord the nerve splits into a dorsal and a ventral root, and the neurons 

 are segregated (Fig. 29.10). The dorsal root contains the afferent neurons, 

 and bears an enlargement, the dorsal root ganglion, which contains 



White mafbcr 

 Dorseil root 



Central 

 -canal 



Dorsal root 

 Oan^lion 



Dorsal 

 ramus 



Vent rail 

 ramus 



VentraJ. 

 root 



Postganglionic fiber 

 ■fco visceral effector 



W hite ramus 

 ^^^communicans 



^ I AV^Fromskin 



Sympathetic \ receptor 

 ganglion \^ , . ^ , 



lo skeletal 

 Postganglion-/|r muscles 

 Fiber To sweat gland 



or cutaneous 

 From, visceral blood vessel 



receptor 



Figure 29.10. A diagrammatic cross section through the spinal cord and a spinal 

 nerve. Each spinal nerve is formed by the union of dorsal and ventral roots, and 

 divides laterally into several branches (rami) going to different parts of the body. The 

 dorsal ramus contains the same types of neurons as the ventral ramus. 



