242 CIRCULATION OF THE BLOOD. 



ready for examination. It is, however, found very advanta- 

 geous to cover those parts of it which do not lie under the 

 microscope with sheets of blotting-paper, for by this means 

 the risk of exposure is diminished, and the undulating move- 

 ments of the water are prevented ; so that the object is 

 rendered much steadier than it would otherwise be. So long 

 as low powers are employed, this arrangement is sufficient ; 

 but if it is desired to use objectives of short focal distance, it 

 is necessary to warm the objective by allowing a stream of 

 water from the same source as that which supplies the stage 

 to pass round it. 



The objects which present themselves to the observer are 

 manifold. Veins and arteries maybe studied of various di- 

 ameters, some of which are free, while others are surrounded 

 by sheaths of tissue in which there are labjTinths of capillaries 

 of surpassing beauty. Several new observations have already 

 been made by this method. One of the most important, phy- 

 siologically, is the fact that the maintenance of the capillary 

 circulation is wonderfully dependent on temperature; and, in 

 particular, that any rise of temperature above the normal is in 

 the highest degree injurious, partly, perhaps, from its direct 

 influence on the blood corpuscles, but mainly because it pro- 

 duces changes similiar to those we have already noticed as 

 occurring in batrachians after long exposure viz., arrest of 

 the capillary blood-stream and escape of the liquor sanguinis 

 and corpuscles into the surrounding tissue. 



46. Artificial Circulation. For many purposes of re- 

 search, it is desirable to observe the circulation independently 

 of the action of the heart. This is accomplished either in the 

 whole body or in an organ, by injecting blood, or a liquid 

 which may be substituted for it, in a constant stream into the 

 arterial system, at the same temperature and under the same 

 pressure as that which natural!}' exists in the arteries. In the 

 case of batrachians, this is accomplished without difficulty, 

 for the temperature of the bod}' differs little from that of the 

 atmosphere, and the nutritive processes can be maintained for 

 long periods, not only without respiration, but without the 

 agent by which oxygen is conveyed to the tissues haemoglo- 

 bin. Consequently the conditions to be observed are very 

 simple. The requirements for the purpose are as follows: 



1. The liquid to be injected ma}' be either serum, defibri- 

 nated blood, or f per cent, solution of chloride of sodium. 

 When, serum is used, it must be absolutely fresh. For this 

 reason, the serum obtained from the slaughter-house is usualty 

 not to be depended upon. It is therefore necessary to use a 

 small rabbit for the purpose. In order to obtain a sufficient 

 quantity of blood from this animal, a canula must be care- 

 fully secured in the carotid, and a clip placed on the artery. 



