124 MANUAL OF HISTOLOGY. 



of a satisfactory chemical analysis. Microscopical examination shows the 

 fluid to be contaminated with the form-elements of mucus. 



REMARKS. 1. Gray "On the Structure and Use of the Spleen" London, 1854, pp. 144 



and 147. Here we find the abundance of colourless elements corroborated. Atten- 

 tion is also directed to the constant occurrence of dark pigment granules, or small 

 elongated crystalline formations, which are occasionally contained in cells. 2. Ac- 

 cording to Gray, loc. cit. p. 152, the blood of the spleen is poorer in cells, but, on the 

 other hand, richer in water, fibrin, albumin, and fat, than other blood. We shall 

 have to discuss, later on, the occurrence in it of other peculiar matters, in consider- 

 ing the organ in question. 



77. 



This is probably the most suitable place to consider more closely 

 the varieties in colour in arterial and venous blood, already men- 

 tioned. 



The colour of the blood (a body-colour) is produced, as we have already 

 seen, by the presence of multitudes of coloured cells suspended in the 

 usually colourless intercellular fluid. Without taking the subordinate 

 differences into account, the tint of arterial blood is a light or cherry red, 

 while that of the veins is of a darker or bluish red tinge (modena). 



The. following is all that is known at present of the cause of these vari- 

 ations in colour. 



It has long been known that certain gases produce a change in the 

 colour of blood. From time immemorial the light tint of the arterial 

 fluid has been ascribed to the action of oxygen, and the darker shade of 

 veins to that of carbonic acid. The correctness of this theory is easily 

 proved by conducting a stream of these gases separately into a quantity 

 of blood. A stream of oxygen causes the latter to become of a light cherry 

 red, and carbonic acid makes it dark. Besides this, blood which has stood 

 for a long time exposed to the air is always much lighter in colour on the 

 surface than deeper down. 



A solution of haemoglobin is similarly affected by a stream of either of 

 these gases. 



But this solution, freed, from formed elements, is transparent; it has 

 the appearance of a " lac colour." 



If we allow blood to freeze, it displays, on being again cautiously thawed, 

 the same transparent colour. The microscope discloses the bodies of the 

 cells, but decolorised. The hemoglobin has passed from them in solu- 

 tion into the plasma. Such lac-coloured blood conducts itself in respect 

 to its colouring properties very similarly to the artificial haemoglobin solu- 

 tion of the chemist, and is exactly the same as the latter after complete 

 destruction of the cells. Its transparency is much greater than that of 

 normal blood with its coloured cells, and it appears with reflected light 

 darker than the latter. 



The more red cells, then, the blood contains, the darker and more opaque 

 is it ; the less it possesses of such elements, the lighter and more trans- 

 parent does it become, seen by transmitted light. 



The form, also, of the cells has a very great influence on the tint of the 

 blood. All agents which cause the red corpuscle^ to shrivel, as, for 

 instance, a concentrated solution of common salt, also render the blood paler 

 in appearance, as seen by reflected light, whilst all reagents which produce 

 an expansion of the cell, such as water, give rise to a darkening of the 

 whole fluid. Such blood increases also in transparency, as might be 

 expected. 



