226 GENERAL ANATOMY OF THE SANGUIFEROUS SYSTEM. 



cava,) were it not that the bronchial arteries anastomose in the 

 lungs with the smaller branches of the pulmonary. 

 The capillary vessels are so abundant, that the width of the 

 spaces left between them are believed in man to be less than the 

 diameter of the vessels themselves. The diameter of the ca- 

 pillaries, which has been taken by the aid of the microscope after 

 the vessels had been minutely injected, has been found to vary 

 in different parts of the bojjy, It appears, however, to be 

 uniform in the vessels of the same organ, but varies in different 

 places from the ^ to ^ parts of an inch. In the brain, the 

 retina and villi, they are even said to measure only from ^ so to 

 w m . The circulation through these vessels goes on slowly, and 

 in an even stream. It appears from microscopical observations, 

 that the current flows more rapidly in the centre, and dimin- 

 ishes gradually toward the periphery of the vessel, so that there 

 is formed on its inner wall, what has been called the motionless 

 layer of blood. By this arrangement the plastic fluid of the 

 blood, (not the blood globules,) is arrested by adhesion, as it 

 were, on the inner surface of the vessel, for the purposes of 

 exosmosis, or transudation into the surrounding substance, which 

 it is to nourish. The mean of the measurement of the capil- 

 laries, according to Miiller, is between the g | gs th and ^ m ih part 

 of an inch. No other tubes in the body are so minute as the 

 capillary vessels, not even those of the kidney or testicle. 



Action of the Heart and Arteries. 



The heart of an adult man, in the middle period of life, con- 

 tracts from seventy to seventy-five times a minute. At each 

 pulsation it is calculated that about two ounces of blood is eject- 

 ed. The whole amount of blood in the human body has been 

 variously estimated, and it is perhaps impossible to arrive at a 

 very accurate conclusion upon the subject ; there is certainly a 

 larger amount in the body than is usually at one time in active 

 circulation, a part of it moving slowly, or remaining stagnant for 

 a time, in the capillaries of some portions of the body. Accord- 

 ing to Wrisberg, one woman lost by a fatal flooding, 26 Ibs. of 

 blood ; and in another plethoric woman, who was beheaded, 



