Biophysical Factors in Drug Action 21 



the membrane lattice. The membrane is heterogeneous or mosaic 

 in structure, but the preponderance of Hpids may confer upon 

 the membrane the properties of a homogeneous oil layer. More- 

 over, the membrane may have a lamellar structure, which 

 possesses peculiar significance according to the particular physio- 

 logical function of the membrane. 



Insect Cuticle as Test Material 



The author ^^' ^^' -^ has found that the study of the uptake 

 of drugs by insects is facilitated by the fact that the cuticle 

 can be readily removed from the insect and studied as a separate 

 physicochemical system. The insect cuticle consists of an outer 

 lipoidal layer which covers a much thicker inner more hydro- 

 philic layer. The outer layer, which is only a few |.i in thickness, 

 contains lipids incorporated in a lipo-protein framework. A 

 proportion of the lipids can be removed by the action of fat 

 solvents. This outer layer, or epicuticle, confers on the cuticle 

 framework its physiological function as a water-impermeable 

 membrane. The inner layer, or endocuticle, may be more than 

 100 |i in thickness, and consists of hydrated protein closely 

 associated with chitin,^'* together with a smaller proportion of 

 lipids. This layer serves a mechanical supporting or exoskeletal 

 function in relation to the internal tissues and body fluids. The 

 cuticle has a pronounced lamellar structure, and the positive 

 form birefringence indicates the presence of orientated lipids in 

 the lamellae, while the extension of the molecules of protein 

 parallel to the cuticle surface is supported by the X-ray diffrac- 

 tion studies of Fraenkel and Rudall,^"* and by the fact that the 

 cuticle can be mechanically separated into component layers. 



Effects of Mixed Drug Systems on Insect Cuticle 



The soft cuticles of blowfly larvae, or ''maggots," are very 

 suitable for experimental manipulation, and they can be attached 

 to small tubes in the form of osmometers. Some very interesting 

 results have been obtained from the study of mixed drug svs- 



