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TEXT-BOOK OF PHYSIOLOGY. 



The structures usually selected for the observation of these phenomena are 

 the interdigital membranes (Fig. 173), the tongue, the lung, the bladder, 

 and the mesentery of the frog. Though any one of these structures will 

 afford an admirable view of the blood-flow, the mesentery for many reasons 

 is the most satisfactory. For a comparison of the phenomena observed in 

 the cold-blooded animals with those in the warm-blooded animals the omen- 

 tum of the guinea-pig may be employed. If the frog is the subject of ex- 

 periment, it should be slightly curarized and the brain destroyed by pithing. 

 The animal is then placed on a small board capable of adjustment to the 



stage of the microscope. The abdo- 

 men is then opened along the side 

 and a loop of intestine withdrawn 

 and placed around a cork ring which 

 surrounds an opening in the side of 

 the frog board. The loop of the in- 

 testine should be so placed that it will 

 lie between the observer and the body 

 of the frog. The mesentery thus ex- 

 posed must be kept moist with nor- 

 mal saline solution. 



When examined with low powers 

 of the microscope, arteries, veins, and 

 capillaries will be found occupying the 

 field of vision. Their general arrange- 

 ment, their size and connections, can 

 be readily determined. After a few 

 preliminary adjustments a region will 

 be found in which the blood is flowing 

 in opposite directions. The vessel ap- 

 parently carrying blood away from the 

 observer is an artery; the vessel appar- 

 ently carrying blood toward the obser- 

 ver is a vein; the smallest vessels are capillaries. The blood in the artery is 

 of a brighter color than the blood in the vein; the blood in the capillaries 

 is almost colorless. The arterial blood-stream not infrequently shows remit- 

 tency, an alternate acceleration and retardation, corresponding to each 

 heart-beat; the capillary and venous streams are uniform and continuous. 

 The relative velocities in the three sets of vessels are indicated by the move- 

 ment of the red corpuscles. In the arteries they pass before the eye so rapidly 

 that they cannot be distinguished ; in the capillaries they pass so slowly that 

 both form and structure may be determined; in the veins, though again 

 moving rapidly, they can often be distinguished. 



The relative positions of the red and white corpuscles in the blood- 

 stream are also apparent; the former occupy the central, the latter the per- 

 ipheral portion, at the same time adhering to the sides of the vessel. Be- 

 tween the axial portion of the stream occupied by the red corpuscles and the 

 wall of the vessel there is a clear still layer of plasma, the result of an adhe- 

 sion of the plasma to the wall. It is this feature which gives rise to the 

 friction between successive layers of the blood-stream, the resistance of the 



Fro. 173. THE VESSELS OF THE FROG'S 

 WEB. a. Trunk of vein, and (b, V) its 

 tributaries passing across the capillary net- 

 work. The dark spots are pigment cells. 

 ( Yeo's ' ' Physiology. ' ') 



