74 PHYSIOLOGY. 



the meshes of which the proper substance of the tissues lies. These 

 minute vessels are called capillaries from their small size, and they 

 may be seen by the aid of the microscope in the web of the frog's foot, 

 (fig. 15,) in the tail of the tadpole, and the lungs, urinary bladder, 

 or tongue of the frog. 



The size of the capillaries is proportioned to that of the red par- 

 ticles of the blood, and can be measured in parts finely injected. 

 Their diameter varies from yogo^^ ^^ -3:7)^00^^ ^^ loon^^ ^^ ^" inch. 

 When filled with blood they are not so much distended as when in- 

 jected, and have seldom been measured when so filled. No other 

 elementary tissues are much more minute than the capillaries. 



Microscopic observations and minute injections have shown that 

 the capillary vessels are merely the fine tubes which form the 

 medium of transition from arteries to veins, and that no other kind 

 of vessels arise from them; that the minute arteries have no other 

 mode of termination than the communication with the veins by means 

 of capillaries ; in a word, that there are no vesseh terminating by 

 open extremities. (MuUer.) Serous vessels^ that is, branches of the 

 blood-vessels too minute to allow the passage of red particles, and 

 consequently traversed merely by the lymph of the blood, may ^05- 

 sibly exist, though they have not been demonstrated. What have 

 probably been mistaken for them, are vessels which are so small as 

 to admit only a single row of blood-corpuscles, the amount of colour- 

 ing matter in which is not sufficient to tinge the light transmitted 

 through them. The existence of vessels in the substance of the cor- 

 nea, which were supposed to be serous or white vessels, is, according 

 to Muller, very doubtful ; they have never been injected. 



The existence of membranous parietes in the capillary system has 

 been doubted by many physiologists, but more accurate research 

 seems to establish the fact that such is the case. The fact that 

 fluids injected into the arteries pass into the veins without extravasa- 

 tion, and that currents cross above and below each other without 

 mixing, have been adduced as arguments for the existence of mem- 

 branous walls. Besides which, Windischmann has dissolved away 

 the other tissues in a delicate membrane found in the ear of birds, 

 leaving the beautiful vascular network, with the meshes empty. 

 Schwann has also discovered by the microscope, that the capillaries 

 have not merely membranous parietes, but a coat in which circular 

 fibres can be distinguished as in the arteries. 



The capillary circulation seems to be in a great degree indepen- 

 dent of the heart's action, since it has been seen to continue in cold- 

 blooded animals after complete excision of the heart. The emptiness 

 of the arterial system after death, although partly owing to the toni- 

 city of the arteries themselves, is commonly more complete than 

 could be thus accounted for, and must therefore be partly due to the 

 capillary circulation. Farther, the process of secretion has also 



