PHYSIOLOGICAL ANATOMY OF THE VEINS. 87 



the intervention of the heart is in cases of acardiac foetuses. Monsters with- 

 out a heart, which have undergone considerable development and which pre- 

 sent systems of arteries, capillaries and veins, have been described. All of 

 these, however, are accompanied by a twin, in which the development of the 

 circulatory system is quite or nearly perfect. 



Influence of Temperature on the Capillary Circulation. Within moder- 

 ate limits, a low temperature, produced by local applications, has been found 

 to diminish the quantity of blood sent to the capillaries and retard the circu- 

 lation, while a high temperature increases the supply of blood and accelerates 

 its current. Poiseuille found that when a piece of ice was applied to the web 

 of a frog's foot, the mesentery of a small warm-blooded animal or to any part 

 in which the capillary circulation can be observed, the number of corpuscles 

 circulating in the arterioles became very much diminished, " those which car- 

 ried two or three rows of corpuscles giving passage to but a single row." The 

 circulation in the capillaries first became slower and then entirely ceased, in 

 parts. On removing the ice, in a very few minutes the circulation regained 

 its former characters. When, on the other hand, the part was covered with 

 water at 104 Fahr. (40 C.), the rapidity of the current in the capillaries was 

 so much increased that the form of the corpuscles could with difficulty be 

 distinguished. 



ClKCULATION OF THE BLOOD IN THE VEINS. 



The blood, distributed to the capillaries of all the tissues and organs by 

 the arteries, is colected from these parts in the veins and carried back to the 

 heart. In studying the anatomy of the capillaries or in observing the passage 

 of the blood from the capillaries to larger vessels in parts of the living organ- 

 ism which can be submitted to microscopical examination, it is seen that 

 the capillaries, vessels of nearly uniform diameter and anastomosing in every 

 direction, empty into a system of vessels, which, by union with others, become 

 larger and larger, and carry the blood away in a uniform current. These are 

 called the venules, or venous radicles. They are the peripheral radicles of 

 the vessels which carry the blood to the heart. 



The venous system may be considered, in general terms, as divided into 

 tsvo sets of vessels ; one, which is deep-seated and situated in proximity to the 

 arteries, and the other, which is superficial and receives the greatest part of 

 the blood from the cutaneous surface. The entire capacity of these vessels, 

 as compared with that of the arteries, is very great. As a general rule, each 

 vein, when fully distended, is larger than its adjacent artery. Many arteries 

 are accompanied by two veins, as the arteries of the extremities ; while cer- 

 tain of them, like the brachial or spermatic, have more than two. Added to 

 these, are the superficial veins which have no corresponding arteries. It is 

 true that some arteries have no corresponding veins, but examples of this 

 kind are not sufficient in number to diminish, in any marked degree, the 

 great preponderance of the veins, both in number and volume. It is impos- 

 sible to give an accurate estimate of the extreme capacity of the veins as 

 compared with the arteries, but it must be much greater. Borelli estimated 



