THE MUSCLES OF THE FLY. 185 



division can be effected, but if it can it does not show that the 

 fibrils are not morphological elements of the tissue ; they are per- 

 fectly distinct, and are sufficiently separated by the interfibrillar 

 ground substance to be clearly differentiated with the highest 

 powers in thin sections. 



Physiologically, these muscles are distinctly differentiated from 

 all other forms of muscle tissue ; their contraction is vibratile, and 

 corresponds to artificial incomplete tetanus. That is, their con- 

 traction may be described as consisting of a prolonged series of 

 single contractions, like those of heart muscle, but with a refrac- 

 tory or fatigue period of extremely short duration. If we regard 

 the interfibrillar material as nutrient material, its abundance 

 between the contractile fibres is explained in relation to the 

 physiological properties of the muscle. 



The last type of muscle which I shall describe is the ordinary 

 muscle of the imago. 



These differ from the muscles of the larva in consisting of a 

 single hollow colonnette ; the cavity is an axial canal, which en- 

 closes a row of nuclei. The fibre is further provided with a more 

 or less distinct investing sheath or myolemma. 



In transverse section such fibres exhibit the same appearances 

 as a field of Cohnheim. The nuclear canal, however, sends off 

 stellate cracks in many preparations, which partially separate it 

 into segments. 



As with the primitive colonnettes of the larval muscle, optical 

 longitudinal sections give variable appearances. I will not decide 

 whether the fibre is divided into transverse segments, each con- 

 taining a nucleus, by the so-called membranes of Krause ; but I 

 am clear that the myolemma adheres very closely to the fibre, and 

 more firmly* between the closely packed nuclei of the central canal 

 than opposite to them, so that it sometimes separates opposite the 

 nuclei, and gives an appearance indicative of Krause's membranes. 

 The fibre exhibits longitudinal striae, the expression of the ultimate 

 fibrils, which appear as bright dots in transverse sections. The 

 dark and light alternate discs are beautifully seen, and rows of 

 bright highly-refractive dots appear in the light discs dividing 

 each into two. 



* In this part of the fibre its demonstration is difficult. M. Viallaines 

 considers it to be less developed and thinner at these points, and adherent 

 to the intermediate (light) discs. 



