139 



CHAPTER III. 



OF THE CIRCULATION. 



L. THE term circulation is applied to that motion by which the blood, 

 setting out from the heart, is incessantly carried to all parts of the body 

 by means of the arteries, and returns by the veins, to the centre whence 

 it began its circuit. 



The uses of this circulatory motion, are to expose the blood, changed 

 by mixing with the lymph and the chyle, to the air in the lungs (respira- 

 tion^) to convey it to several viscera, in which it passes through different 

 steps of purification (secretions /) and to send it into the organs whose 

 growth is to be promoted, or whose losses are to be repaired, by the nu- 

 tritive and acimalized part of the blood brought into a state of perfection 

 by these successive processes (nutrition.'] 



The circulatory organs are less useful in elaborating, than in convey- 

 ing the fluids. To form a just conception of their uses, one may com- 

 pare them to those workmen in a large manufactory in which various 

 kinds of goods are made, who are employed in carrying the materials 

 to those who are to work them; and as among the latter, some finish the 

 work, while others prepare the materials, so the lungs to the secretory 

 glands are continually occupied in separating from the blood whatever is 

 too heterogeneous to our nature to become assimilated to our organs, or 

 to afford them nourishment. 



To understand, thoroughly, the mechanism of this function, it is neces- 

 sary to study separately the action of the heart, that of the arteries which 

 arise from it, and, lastly, that of the veins which enter it. The union of 

 these three classes of organs, forms the circle of the circulation. 



LI. Of the action cf the heart. In man, and in all warm-blooded ani- 

 mals, the heart is a hollow muscle, the inner part of which is divided into 

 four large cavities which communicate with one another ; from these, 

 vessels arise which convey the blood to all parts of the body, and the ves- 

 sels which bring it back from all those parts, likewise terminate in these 

 cavities. 



The heart is placed in the chest, between the lungs, above the dia- 

 phragm, whose motions it follows; it is surrounded by the pericardium, 

 a dense and fibrous membrane admitting of very slight extension, closely 

 united to the substance of the diaphragm, covering the heart and great 

 vessels, without containing them in its cavity, furnishing an external co- 

 vering to the heart, and bedewing its surface with a serous fluid, which 

 never accumulating, except in disease, facilitates its motion, and prevents 



An admirable series of experiments were subsequently performed by Dr. J. O'B. 

 Lawrance, and Dr. B. II. Coates, in which they very clearly establish, that articles 

 taken into the stomach may escape by three outlets for absorption. By the vena portae, 

 the cesophageal veins, and the thoracic duct ; and if all these be closed, the absorbed 

 matter no longer finds its way into the circulation or urine. See Phil. Med. Jour. vol. 

 v. p. 327 Godman. 



See APPENDIX, Note Q. 



