364 



omentum, and had already described it in connection with 

 another inquiry. The omentum of the guinea-pig is a 

 membrane of extent relatively comparable to that of man, 

 but its structure is entirely different. First it is attached, not 

 to the transverse colon, but to the greater curvature of the 

 stomach. Secondly, it consists, not of four layers of mem- 

 brane, but two ; and lastly, it contains very little fat, but in 

 place of it a great quantity of cells, which ai'e collected in a 

 peculiar way about the blood-vessels and in their neighbour- 

 hood, partly in the form of perivascular sheaths, partly in the 

 form of little collections or nodules consisting of cells lying 

 in the meshes of a plexus of capillaries. 



Hence, from the simplicity of its anatomical relations, and 

 particularly from its being attached on one side only to the 

 stomach, in which respect no membrane is coinparable with 

 it ; from its perfect transparency ; from its abundant vascu- 

 larity ; and from its containing not only vessels, but living cells, 

 and these cells of two kinds, namely, epithelial and parenchy- 

 matous, it is obvious that the omentum of the guinea-pig 

 offers a splendid field for observation. 



We have now only to speak of the mechanical arrangements 

 which are necessary in order that the omentum may be placed 

 under the microscope under natural conditions. It is to be 

 borne in mind that the guinea-pig is a warm-blooded animal, 

 that the omentum is part of the peritoneum, that the tissue 

 is extremely delicate, and readily becomes inflamed when 

 touched. 



Hence for observation the membrane must be immersed — 

 immersed, however, not in water, for water would at once 

 irritate and kill the tissue, but in solution of common salt of 

 proper strength. Such a solution is what physiologists call 

 an indifferent fluid, because, when it comes in contact with 

 living cells, it does not appreciably interfere with their 

 vital processes. Secondly, it cannot be in a natural condition 

 unless it retain the temperature of the living body. The 

 arrangement for securing this is somewhat complicated. The 

 membrane is laid out in a glass dish, which is supported on 

 the stage of the microscope by a hollow brass plate, through 

 Avhich a stream of water flows at a rate and temperature so 

 regulated that the dish and its contents are maintained at a 

 temperature closely agreeing with that of the body. 



For commencing the observation this is all that is ne- 

 cessary. If, however, it is continued, the observer soon 

 encounters two difficulties, both of which must be overcome. 

 The one arises from the clouding of his objective when it is 

 brought near the warm surface of the saline solution ; the 



