PECULIARITIES IN THE VENOUS CIRCULATION. 683 



the combined areas of the two venae cavse are greater than the area of 

 the aorta. 



The rate of motion of the blood in the veins, is more subject to dis- 

 turbing causes, whether of acceleration or retardation, than in any 

 other part of the circulation. Thus, the effects of muscular pressure, 

 though, on the whole, favorable to the onward flow of the blood in the 

 veins, are necessarily intermittent, according or not as the muscles are 

 at play. Again, the opposite influences of expiration and inspiration, 

 though felt only within a certain distance of the thorax, and so affect- 

 ing the rate of motion of the blood in the large veins only, are them- 

 selves liable to great variations, according to the activity, violence, or 

 depth of the inspiratory or expiratory movements. Such variations 

 occur constantly during life, and incessantly alter the rate of motion 

 of the venous blood-current. In experiments on animals not subjected 

 to the continued and uniform influence of chloroform, the struggles, 

 and the respiratory efforts of the creature, greatly disturb the velocity 

 of the venous current, sometimes checking, sometimes accelerating it. 

 Individual estimates of the velocity of the blood in the veins, must 

 therefore be accepted with some reservation. 



There are certain peculiarities in the venous circulation of particular 

 parts of the body. Thus, the portal circulation is peculiar, from the 

 fact that the blood passes in it, through a second capillary network, 

 before it returns to the heart ; for the blood which circulates thus 

 through the liver, has already been driven through the capillary ves- 

 sels of the other abdominal organs of digestion. There are no valves 

 in the portal or hepatic veins ; but the latter are retained constantly 

 in a pervious state, by their adhesion to the substance of the liver, a 

 condition favorable to the passage of the blood from that organ. 

 Again, the circulation within the cranium presents peculiarities ; the 

 arterial trunks which enter it, four in number, are of large size, trav- 

 erse bony passages in their way to the cranial cavity, and unite by 

 anastomoses in the interior of the skull, at the base of the brain, 

 all which arrangements are calculated to secure a full and free supply 

 of blood to the brain, under various conditions of external pressure, or 

 other impeding causes. Besides this, the proper arteries of the brain 

 ramify, in an unusually tortuous manner, upon its complex surface, 

 and at last divide, in the pia mater, into a close web of numberless 

 branches supported by a delicate cellular tissue ; from these, long 

 slender minute vessels enter the brain at all points, insuring a perfect 

 supply of blood, and its even and gentle entrance into the delicate 

 cerebral substance. The veins within the cranium present special 

 modifications ; first, they have no valves ; moreover, the largest venous 

 channels consist of passages between layers of the dura-mater, the 

 fibrous membrane which immediately lines the skull ; hence, they are 

 not subjected to accidental pressure, such as might interfere with the 

 blood-current within them. Lastly, as the cranium itself has un- 

 yielding walls, the circulation of the blood through the brain is carried 

 on under very peculiar conditions, as compared with that of other 

 organs, which are subject to atmospheric, and perhaps muscular pres- 

 sure also. The brain and the blood being incompressible, the quantity 



