THE PHYSIOLOGY OF THE CIRCULATION. J397 
ness ; and that every moment of our lives, both during our 
waking and our sleeping hours ? 
Harvey thought the heart was the only organ conducting the 
circulation ; but it has been shewn that the circulation may goou 
without a heart, its existence being only necessary when organi- 
zation becomes voluminous, and structure complex. Arteries 
terminate principally in veins, capillary and secerning tubes, in 
cells and on surfaces. Their course is generally well protected 
against injury. They are not so numerous as veins, nor do they 
anastomose so freely. They have no valves, except at their 
origin from the heart. The blood in them is a bright scarlet, 
with the exception of the pulmonary, which performs the office 
of a vein, and in which, at its origin, the blood is the darkest in 
the body. Thus far we see the blood is propelled to all parts of 
the body by the combined action of the vital principle, the heart 
and arteries. Its return, which is not so easily explained, is effect- 
ed by the veins, which are thin elastic tubes, possessed of two 
coats essentially. The external appears a mixture of elastic and 
muscular fibres, and the internal a serous coat, taking their rise 
from cells, sinuses, surfaces, but principally from the minute ter- 
minating capillaries of the arteries. They are far more numerous 
than arteries, anastomose very freely, generally have valves, and 
are rather superficial in their course. The circulation in them 
appears to be influenced by many circumstances ; first, and most 
important, the vital principle, next the impetus from the contrac- 
tion of the heart and arteries ; the equal pressure on all sides 
from muscular action, &c., assisted by the valves in preventing 
the return of the blood, and becoming larger from their origin, 
thus presenting less obstruction to its passage. Respiration has 
also a material effect. In inspiration the lungs are in the best 
state for the passage of the blood, their distention with air 
straightening the vessels, thus allowing a free passage ; whereas 
in expiration the collapsed state of the lungs makes the vessels 
more tortuous and compressed, so as in some degree to retard it. 
The free passage of the blood through and into the lungs in in- 
spiration, produces a partial vacuum in the right ventricle of the 
heart, into which the blood from the right auricle immediately 
rushes from the large veins into the auricle, and from the smaller 
branches into the larger. These circumstances connected, 
appear to me to account satisfactorily for the circulation of the 
blood to and from the heart. The formation of blood com- 
mences in the stomach and intestines, and is completed in the 
lungs. 
[To be continued.] 
