THE NERVOUS CONTROL OF THE BLOOD VESSELS 1039 



often taking a separate course as the lesser splanchnics. The fibres can be 

 seen to pass through the sympathetic chain of the thorax without inter- 

 ruption, and for the most part have their cell station in the large ganglia, 

 especially the semilunar ganglia, of the solar plexus, whence a thick mesh- 

 work of non-medullated fibres is distributed along all the vessels of the 

 abdominal viscera. The area of the vessels innervated by this nerve is so 

 large that section of this nerve on each side causes a considerable fall in the 

 general blood pressure. This fall is more marked in animals such as the 

 rabbit and other herbivora, in which the alimentary canal is proportionately 

 very much developed and has a correspondingly large blood supply. 



VASO-DILATOR NERVES 



Since the arteries are in a constant condition of moderate contraction, 

 a dilatation might be brought about by a relaxation of this tone by an 

 inhibition of the normal constrictor impulses proceeding to the vessels from 

 the vaso-motor centre. We find however in many parts of the body 

 evidence of the existence of a nerve supply to blood vessels antagonistic 

 in its function to the vaso-constrictors. Thus, if the chorda tympani nerve 

 going to the submaxillary gland be cut, no change is evident in the blood 

 vessels of the gland. But if its peripheral end be stimulated, there is instantly 

 free secretion of saliva from the gland, and all the blood vessels are largely 

 dilated. In consequence of this dilatation the blood rushes through the 

 capillaries so quickly that it has no time to lose much of its oxygen; the 

 blood flowing from the vein is therefore bright arterial in colour, and is 

 increased to six or eight times the previous amount. If atropine be injected 

 into the animal, the action of the chorda tympani on the blood vessels is 

 unaffected, although the secretion on stimulation is abolished. The chorda 

 tympani is therefore said to contain vaso-dilatqr fibres for the vessels of the 

 submaxillary gland. Other examples of vaso-dilator (or dilatator) nerves 

 are the small petrosal nerve to the parotid gland, the lingual nerve to the 

 blood vessels of the tongue, and the nervi erigentes or pelvic visceral nerves 

 to those. of the penis. 



The course of these typical dilator nerves differs widely from that of the 

 constrictors. Whereas the latter leave the central nervous system over a 

 limited area of the cord, the vaso-dilators take their origin together with 

 any of the cerebro-spinal nerves. Thus the chorda tympani fibres, and 

 probably those contained in the petrosal nerve, arise from the nervus inter- 

 medium between the seventh and eighth cranial nerves. The nervi erigentes 

 leave the lower end of the cord by the anterior roots of the second and third 

 sacral nerves. All of them, like % the vaso-constrictors and probably all 

 visceral nerve fibres, are interrupted by ganglion cells before reaching to 

 their destination. These cells however lie, not in the lateral chain of the 

 sympathetic, with which the nerves have no connection at all, but peri- 

 pherally, and are generally embedded in the organs to which the nerves are 

 distributed. Thus the chorda tympani fibres to the submaxillary glands are 



