524 A MANUAL OF VETERINARY PHYSIOLOGY 



The term ' sympathetic ' was employed by the earlier physi- 

 ologists to describe the carrying out of certain ■ vegetative ' 

 functions of the body, which were not within the knowledge or 

 control of the animal. They employed the term in much the 

 same sense that the word ' reflex ' is employed to-day ; but the 

 work of Gaskell and Langley has remodelled the entire con- 

 ception of the sympathetic system, and even changed its name. 

 Langley proposes to call it the autonomic system, in order to 

 indicate that it possesses a certain independence of the central 

 nervous system, with power of self-government, and this term 

 is adopted by modern physiologists. It has the further advan- 

 tage of including certain branches of the cranial nerves, which 

 function as sympathetic, though of cerebral origin. Gaskell 

 divides the sympathetic system into three parts : first, the chain 

 of ganglia running on either side of the spine, closely attached 

 to the arches of the ribs in the thoracic region, and extending 

 back to the sacrum. This he calls the vertebral or lateral ganglia. 

 The second part of the sympathetic system is that consisting of 

 the large ganglia in front of the vertebrae, in the upright animal, 

 known as prevertebral ganglia, and represented by such large 

 masses of nervous tissue as the anterior and posterior mesenteric 

 ganglia ; while the third group lies beyond these in the walls of 

 the tissue — for example, the intestines — and is known as terminal 

 ganglia (Fig. 157). These three groups are linked together, 

 while the first group is linked up with the spinal cord. Langley 

 has shown that there is no essential difference in function between 

 the vertebral and prevertebral ganglia, but a great difference 

 between these and the terminal ganglia. 



The fibres of the autonomic system are mainly efferent, and 

 as derived from the spinal cord are medullated in structure and 

 very narrow. After they have passed through a ganglion they 

 generally lose the medulla, and become non-medulla ted. In 

 consequence there are white fibres and grey fibres in the auto- 

 nomic system, depending upon whether they have lost or re- 

 tained their medullary sheath. There is no difference in the 

 structure of these from similar nerves studied in the cerebro- 

 spinal system, with the exception that the fibres are much finer. 

 The ganglia consist of multipolar cells, which do not differ struc- 

 turally from similar cells met with in the parent system ; but their 

 physiology is wholly different. They seem incapable of sum- 

 mation, inhibition, and irreversibility of conduction. It was the 

 presence of multipolar cells which at one time led to the belief 

 that the ganglia of the autonomic system were centres of reflex 

 action. There are no reflex actions in the autonomic system ; 

 it is worked as a reflex effect — viz., not under the control of the 

 will — but it has no power to originate reflex functions, because 



