716 MEDICAL ENTOMOLOGY 



After a few preliminary dissections to get an idea of the general rela- 

 tions of the parts, the exo-skeleton, and any internal processes which may 

 arise from it should be studied, in order to ascertain the route by which 

 the required part can be most easily got out. Usually the larger part of 

 the alimentary canal can be drawn out backwards, by separating the last 

 segment from the penultimate one. The ovaries, if they do not contain 

 large ova, will usually come out by the same route. But neither a dis- 

 tended gut nor a mature ovary will come out unharmed in this way, and 

 one has to make more space by cutting up one side of the abdominal 

 wall. Salivary glands will come out of the thorax if the head is pulled 

 off in many cases, but cannot be expected to do so if they pass down to 

 the abdomen and have a large tracheal supply. The proventriculus of 

 Tabanus will not come out of the thorax backwards on account of 

 the small aperture left by the mesophragma. In ticks the only way 

 to get at any of the organs is to open up the dorsal wall. In 

 every case the method of dissection has to be decided on from a con- 

 sideration of the anatomy of the parts. Where the gut is a distensible 

 organ, as in the case of Tabanus, if it should be necessary to dissect it 

 out when full there is nothing for it but to dissect off the integument all 

 round it, for otherwise it is certain to rupture. If mature ova are 

 required, all that is necessary in most cases is to make a slit in the. side 

 of the abdominal wall, and to press gently with a needle. One or two 

 of the ova will then slip out. In any case, if the ova are evidently 

 mature, it is advisable to let some of them out in this way before attempt- 

 ing to pull out the gut. 



The actual operation of dissection .is so much a matter of practice 

 that one cannot do more than offer a few suggestions. In the first place 

 always use a black background for the dissection of internal organs, and 

 have as good a light as possible coming from above. If any muscle gets 

 torn, as it always does in the dissection of the thorax, or if an organ is 

 ruptured, flood the preparation with saline, to wash away the milky fluid. 

 Suck up the saline again with a fine pipette, and add some fresh solution 

 Always remove from the slide all structures not further required, to pre- 

 vent confusion. Tracheae are recognized at once by their silvery- 

 appearance, and should be cut across with the needle, as they hold the 

 other parts in position. Use either as small a quantity of saline as will 

 keep the tissues moist, or else as much as the slide will hold ; with any 

 intermediate quantity the surface tension is a source of trouble, as it 

 causes the organs to follow the movements of the needle in a most an- 

 noying manner. 



