546 OF THE CIRCULATION OF BLOOD. 



sets of capillaries, between its ejection from the heart, and its return to it. It 

 is first transmitted through the respiratory capillaries, for the purpose of aera- 

 tion ; the confluent vessels, which collect the arterial blood from these, ter- 

 minate in the general systemic trunk or Aorta, in which, as in the veins of 

 Man, there is an absence of pulsation, and by these, it is distributed to the 

 systemic capillaries ; and the blood which, after passing through these, re- 

 turns from the posterior part of the body, and from the viscera, passes through 

 another set of capillaries, those of the liver and kidneys, before it returns to 

 the heart. Even in the warm-blooded Vertebrata, in which the respiratory 

 circulation is separately performed, the blood which is returned from the in- 

 testines, passes into a trunk, the Vena Portae, which again subdivides into 

 capillary ramifications, being transmitted over the plexus of biliary ducts, of 

 which the liver is chiefly composed ; and thus the Vena Portae, as Hunter 

 justly observed, should be considered rather in the light of an artery,* re- 

 sembling as it does the aorta of Fishes. Considering the small amount of 

 pressure which is exerted by the blood, upon the sides of the vessels that are 

 formed by the reunion of capillaries, it seems impossible to imagine that the 

 vis a tergo derived from the impulsive action of the Heart, can be alone suf- 

 ficient to maintain the portal circulation. 



2. Action of the Heart. 



717. The Heart is endowed in an eminent degree with the property of 

 irritability ; by which is meant, the capability of being easily excited to 

 movements of contraction alternating with relaxation ( 574). Thus, after 

 the Heart has been removed from the body, and has ceased to contract, a 

 slight irritation will cause it to execute, not one movement only, but a series 

 of alternate contractions and dilatations, gradually diminishing in vigour until 

 they cease. The contraction begins in the part irritated, and then extends to 

 the rest. It appears from Mr. Paget's experiments,! that it is necessary for 

 the propagation of this irritation, that the parts should be connected by mus- 

 cular tissue, of which a very narrow isthmus will suffice ; and that the pro- 

 pagation will not take place, if the connecting isthmus be composed of ten- 

 don, even though this be a portion of the auriculo-ventricular ring, which has 

 been supposed by some to be peculiarly efficacious in this conduction. That 

 the irritability of the heart is not dependent upon the Cerebro-spinal system, 

 appears not merely from the manifestation of it, when the organ is altogether 

 removed from the body, but also from the fact, that if the flood of blood 

 through the lungs be kept up by artificial respiration, the heart's action will 

 continue for a lengthened period, even after the Brain and Spinal Cord have 

 been removed, and when animal life is, therefore, completely extinct. Hence 

 we see that the Irritability of this organ must be an endowment properly be- 

 longing to it, and not derived from that portion of the Nervous System. Like 

 the contractility of other muscles, it can only be sustained for any great length 

 of time, by a supply of Arterial blood to its own tissue ( 584). It is much 

 less speedily lost in cold-blooded animals, however, than in warm-blooded ; 

 the heart of the Frog, for example, will go on pulsating for many hours after 

 its removal from the body ; and it is stated by Dr. MilchellJ that the heart of 

 a Sturgeon, which he had inflated with air, continued to beat, until the auricle 



* That it conveys venous blood, is no reason to the contrary; since this is the case with 

 the pulmonary artery. The character of an artery is derived iroiu the. division of its current 

 into several divergent streams. 



f Brit, and For. Mod. Review, vol. xxi. p. 551. 



J American Journal of the Medical Sciences, vol. vii. p. 5S. 



