238 THE CIRCULATION OF THE BLOOD 



These are provided by the sympathetic chain and its branches (Fig. 218). 

 The vasoconstrictor fibers in the anterior spinal roots leave the latter 

 by way of the corresponding white rami communicantes, and pass into 

 the neighboring sympathetic chain, along which they either ascend or 

 descend, according to their ultimate destination. In their course they 

 come into contact with the sympathetic ganglia, through one or two of 

 which they may pass without any change, but ultimately each fiber ar- 

 rives at some ganglion, in which it terminates by forming a synapsis 

 around one of the ganglionic nerve cells. The axon of this nerve cell 

 then continues the course by the nearest gray ramus commuhicans back 

 to the spinal nerve beyond the union of its anterior and posterior roots. 

 Up to the point where the fiber forms a synapsis with a ganglionic nerve 

 cell, it is medullated and is known as the preganglionic fiber. Beyond 

 the nerve cell, it is nonmedullated and is known as postganglionic 

 (page 894). 



The exact ganglion in which a given vasoconstrictor fiber becomes connected with a 

 nerve cell can be determined by the nicotine method of Langley. Local application to 

 the ganglion of a weak solution of this drug (1 per cent) paralyzes the synaptic con- 

 nection, so that a stimulus applied to the preganglionic fiber no longer produces its 

 effect. Suppose, for example, that a vasoconstrictor fiber has been found by the stimula- 

 tion method to travel through several ganglia, and we wish to determine in which of 

 these the synapsis occurs : we can do so by applying the stimulus at a point central to 

 the ganglia after painting each of them in turn with the nicotine solution. If the 

 application of the drug to a given ganglion is found to cause no alteration in the 

 effect produced by stimulation, then we know that there can not be any synaptic 

 connection in that ganglion, and we proceed in the same way till we have located 

 the ganglion in which synapsis occurs. It is important to remember that the post- 

 ganglionic vasoconstrictor fibers in a gray ramus communicans do not coine from the 

 preganglionic fibers of the corresponding spinal root, but from fibers coming through 

 white rami at a higher or a lower level. 



The above description applies to the vasoconstrictor fibers proceeding to the vessels of 

 the anterior and posterior extremities, those for the former arising (in the dog) from 

 about the fourth thoracic to the tenth; and those for the latter, from the lowest thoracic 

 and the first three lumbar nerve roots. The cell station for the fibers to the fore limbs 

 is in the stellate ganglion, and for the hind limbs in the last two lumbar and first two 

 sacral ganglia of the 'abdominal sympathetic chain. 



The vasoconstrictor fibers to the vessels of the head and neck run a somewhat dif- 

 ferent course, there being no convenient cerebrospinal nerve along which the post- 

 ganglionic fibers may run. The fibers to the blood vessels of the head leave the cord 

 by the second to the fourth or fifth thoracic roots and pass by the corresponding white 

 rami communicantes into the sympathetic chain, up which they run, passing through the 

 stellate ganglion, the ansa subclavii, and the inferior cervical ganglion, then ascending 

 in the cervical sympathetic to the superior cervical ganglion, where their cell station 

 exists. The postganglionic fibers on leaving this ganglion travel to their destination 

 mainly along the outer walls of the blood vessels. 



The vasoconstrictors to the abdominal viscera are carried by the splanchnic nervea, 

 the fibers of which come off from the lower seven thoracic and the uppermost lumbar 



