284 STRUCTURE OF THE VERTEBRATES 



sweat glands, the muscles of the cutaneous capillaries, and the 

 minute muscles of the hairs. Other fibers from the vertebral 

 ganglia extend ventrally into the mesentery to the small pre- 

 vertebral ganglia, and take impulses to and from the glands 

 and smooth muscles of the viscera. 



Although the autonomic system is not under "voluntary" con- 

 trol, it is intimately connected with the central nervous system 

 and influenced by it. Any pain sensation in the deeper viscera 

 is transmitted to the brain; and conversely, somatic sensory 

 sensations may be transferred over to the visceral motor tracts. 

 Motor stimuli from the brain may be sent to the autonomic 

 ganglia in response to pain, visual or auditory sensations; and 

 in turn cause cutaneous or visceral reactions. The lungs, heart 

 and upper viscera are innervated by autonomic and also by 

 cranial nerve fibers. Functionally the latter may be termed 

 parasympathetic fibers, although they are antagonistic in their 

 action to those of the autonomic ganglia. Cranial motor fibers 

 in the viscera are usually depressors of action while the autonom- 

 ic impulses are accelerators, and any imbalance between the 

 two types of stimuli tends to cause a physiological upset in the 

 organs affected. 



Let us summarize the reactions. A sensation received in the 

 skin through a receptor will be transferred over to a somatic 

 sensory fiber to the cord. At this point it will be sent in two 

 directions: (1) directly to a spinal motor nerve, and (2) to the 

 brain. The reflex stimuli will go immediately from the cord to 

 the nearby effectors; and following them, voluntary impulses 

 will travel from the brain to the motor nerves. The time differ- 

 ence between reflex and voluntary movement is measurable; 

 and in an animal such as the giraffe, with short legs and a long 

 neck, a sensation received in the hind foot would return as a 

 motor impulse from the cord before the sensory stimulus had 

 reached the brain. 



A sensory impulse may be, and usually is, transferred to the 

 visceral motor fibers of the cord and sent to the vertebral 

 ganglia. From this point motor impulses would be sent out to 

 the deeper viscera and the somatic tissues innervated by these 

 autonomic fibers. Pain sensations from the viscera travel to the 

 vertebral ganglia, and from them over two pathways: (1) 



