996 PHYSIOLOGY 



The constrictor impulses to the hind limb in the dog arise from the 

 nerve-roots between the eleventh dorsal and third lumbar roots. All 

 the fibres end in connection with cells in the sixth and seventh lumbar 

 and first and second sacral ganglia of the sympathetic chain, whence 

 the impulses are carried by grey rami to the nerves making up the sacral 

 plexus. 



The most important vaso-motor nerve of the body is the splanchnic 

 nerve. This nerve receives most of the fibres forming the white rami from 

 the lower seven dorsal and upper two or three lumbar roots, the latter fibres 

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

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

 tion, 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 large 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 consequently a very 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 in- 

 creased 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 inter- 



