NERVOUS CONTROL OF THE BLOOD-VESSELS 1121 

 thorax without interruption, and for the most part have their cell- 

 station in the large ganglia, especially the semilunar ganglia, of the 

 solar plexus, whence a thick meshwork 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 large fall in the general blood pressure. 

 This fall is more marked in animals such as the rabbit and other herbi- 

 vora, in which the alimentary canal is proportionately very much 

 developed, and has consequently a very large blood-supply. 



VASO-DILATOR NERVES 



Since the arteries are in a constant condition of moderate con- 

 traction, 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 peri- 

 pheral 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- 

 dilator 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 intermedius 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 sympathe f: 



71 



