XII.] THE DIRECTION OF DEVELOPMENT. 141 



a most interesting instance of this, that the amjjMoxus, a AmpH- 

 species of fish which is the lowest of all vertebrates, is at °^^' 

 once "without a brain, though it has a spinal cord, and 

 without a distinct heart, instead of which it has several 

 pulsating enlargements of the blood-vessels.^ These two 

 characters make it quite unique among vertebrates, and 

 probably show that it is a link between the vertebrates 

 and some now lost class of low organization. 



The parallelism between the cii'culatory and nervous Blood- 

 systems is further shown in this, that those parts of the ^^^^^ ^re 



body which are the most abundantly supplied with blood- abundant 



, , 1 1 ,1 T 1 -ii inthesame 



vessels are also the most abundantly supplied with nerves ; places, 



and the nails and hair, which have no supply of blood at 



all, are also without nerves.^ The activity of the two is Their 



flption. IS 



heightened or lowered together. The vessels of inflamed heightened 

 parts are unusually full of blood, and the action of their together, 

 nerves is heightened : this last is proved by increased Inflamma- 

 sensibility, and also, as I have shown reason for believing, 

 by elevated temperature.^ And, what is a parallel fact to 

 this, the blood-vessels of the brain are comparatively empty 

 during sleep, which essentially consists in a lowered action Sleep, 

 of the brain ; and the same is true of the blood-vessels of 

 the retina.* It is possible to lower the nervous activity so Want of 

 as to destroy the sensibility of any part, by tying the causes in°- 

 arteries so as to deprive it of its supply of fresh blood.^ It sensibility, 

 belongs to this class of facts that the heart, which is the Connexion 

 central circulatory organ, is more easily acted on by nervous ^ith^^ 

 influence than any of the other organs of the vegetative brain, 

 life.^ The susceptibility of the heart to influences arising 

 from the emotions has caused it, indeed, in popular 

 laneuage, to be regarded as their seat. 



We have seen how dependent is the activity of the Depend- 



6I1C6 of 



nervous system on the supply of blood. But the converse nervous 

 is not true : the activity of the circulating system does not action on 



1 Carpenter's Comparative Physiology, p. 447. 



2 Carpenter's Human Physiology, p. 604. 



3 Ibid. p. 99. * Ibid. p. 590. 

 * Carpenter's Comparative Physiology, p. 175. 



6 Claude Bernard, in Revue des Deux Mondes, 1st March, 1865. 



