REPORT ON ANIMAL PHYSIOLOGY. 131 



that the phaenomena of local inflammation, of determination of 

 blood to particular regions, the emptiness of the arteries after 

 death, &c., are to be explained*. It may be abnormal, as in 

 many of these instances. It may also be suspended or destroyed. 

 Nervous influence seems to be a condition of its continuance, 

 where the animal has nerves. It is stated that when the main 

 artery of a limb is tied, the circulation is not maintained by 

 anastomosing branches if the nerve be also tiedf. Many have 

 recorded that in persons bled to fainting, the blood which last 

 issues from the vein is arterial in colour. 



The capillary vessels themselves seem to contribute nothing 

 to the motion of the blood : the diameter of the small threads of 

 blood in any of them is not seen to be in the least variable, whilst 

 the heart's action continues the same and the muscles are at rest. 

 They are intermediate between the minute arteries and the minute 

 veins, and continually anastomose. Dr. Marshall Hall states that 

 he has in vain sought for instances of the immediate termination 

 of a minute artery in a minute vein : they all first pass into 

 capillaries. In this way it is that the blood is brought into near 

 contact with the parenchyme of the various tissues, and the pro- 

 cesses of nutrition effected. It was the opinion of the older 

 physiologists that all the minute arteries did not thus terminate, 

 but that some ended by open mouths, thus allowing the blood 

 itself to enter into the composition of the different organs of the 

 body. If, however, the transparent parts of living animals be ob- 

 served, all the particles of blood are observed to pass from the 

 minute arteries through the network of the capillaries, and by 

 these to enter into the veins. The minute anatomy of glands also 

 shows that the last divisions of their ducts are closed tubules ; 

 that on these the capillary vessels are largely distributed, but 

 never pass into them. The smallest visible capillaries are those 

 which convey red particles of the blood in single files ; their dia- 

 meter therefore is about the :jy\y^dth part of an inch. That there 

 are others so small as merely to convey the lymph of the blood 

 is however the opinion of many physiologists. It was the opi- 

 nion of Haller, and Bichat, and Bleuland that there are such 

 vessels ; the contrary that of Mascagni and Prochaska, and of 

 Soemmerring in his later writings. The primitive fibres both of 

 muscle and of nerve are smaller, considerably, than the least vi- 

 sible capillaries. 



In the lowest animals, this vital attraction between the mole- 

 cules of their body and the nutrient fluid which penetrates, by 

 imbibition, their uniform mass, appears to be sufficient for the 

 purposes of their oeconomy. But in higher creatures, a moving 



* Kalken Brenner, Majendie's Journal, viii. 81. t Baumgsertner. 



K 2 



