248 SUMMARY. 



apparent, during their active healthy functions, in inflammatory diseases, and after the 

 use of particular medicines. On a minute injection after death, the ganglia are very 

 vascular, but this is not their ordinary appearance. 



The supply of blood is regulated by the nerves during the action either of a single 

 organ, or of the whole body, for the heart itself can only circulate it generally by a 

 uniform impulse over the system ; and without this influence of the nerves, any change 

 in the action of an organ would misdirect it, and produce confusion. The nerves and 

 bloodvessels, by ministering to the functions of each other, preserve the balance of 

 both, the impulse of the blood exciting the nerves, and these the contractility of the 

 coats of the vessels, and preventing over-distension. Any moderate excitement may 

 be confined to the nerves and bloodvessels of the part, but one more extensive is 

 communicated generally to the nervous and sanguineous systems. 



Some of the veins are accompanied by nerves, and particularly those underneath 

 the skin of the extremities by the cutaneous, and in the inferior vena cava of mammalia 

 by the right phrenic. In birds the par vagum accompanies the jugular vein instead of 

 the carotid artery, as in mammalia. The veins are scantily supplied with nerves, but 

 when they furnish blood for secretions, or other important purposes, they receive a 

 more copious quantity, as when filaments from the hepatic plexuses supply the 

 branches of the vena portse. It is probable that all the veins are influenced by the 

 state of the nerves, and become over-dilated, not only by any obstruction to the return 

 of blood at the viscera, but by the want of tone, which a diminished nervous power 

 produces ; and as the pores of the skin are relaxed in some instances from the same 

 cause, profuse perspiration may take place. It is, however, probable that several 

 nerves accompany veins, more on account of their safety than for the performance of 

 any specific function, and that the par vagum accompanies the jugular vein in birds, 

 that it may have a more free position than if it had accompanied the carotid artery in 

 the more limited receptacle upon the middle of the spine ; but by adhesion to the vein 

 it is secure from the action of the oesophagus in conveying the food, because the force 

 of the pressure falls principally on the returning blood. When, however, the veins 

 assume the place of arteries, nerves accompany them for the same purpose they do the 



