104 C. P. MARSHALL. 



sources of heat of the body, and are, "par excellence, the 

 thermogenic tissues." 



It thus appears that the thermogenic function of muscle 

 absorbs a far greater amount of its energy than does the con- 

 tractile function, and if we attribute the thermogenic func- 

 tion to the sarcous substance and the contractility to the net- 

 work, the above objection appears to receive a satisfactory 

 answer. 



The following quotation from Prof. Michael Foster 1 is 

 curiously in accordance with the view of the structure and 

 function of muscle maintained above, and may fitly conclude 

 this paper. 



" It is quite open for us to imagine that in muscle, for 

 instance, there is a framework of more stable material, giving to 

 the muscular fibre its histological features, and undergoing a 

 comparatively slight and slow metabolism, while the energy 

 given out by muscle is supplied at the expense of more fluc- 

 tuating molecules, which fill up, so to speak, the interstices of 

 the more durable framework, and the metabolism of which 

 alone is large and rapid." 



Summary. 



1. In all muscles which have to perform rapid and frequent 

 movements, a certain portion of the muscle is differentiated to 

 perform the function of contraction, and this portion takes 

 on the form of a very regular and highly modified intracellular 

 network. 



2. This network, by its regular arrangement, gives rise to 

 certain optical effects which cause the peculiar appearances of 

 striped muscle. 



3. The contraction of the striped muscle-fibre is probably 

 caused by the active contraction of the longitudinal fibrils of 

 the intracellular network ; the transverse networks appear to 

 be passively elastic, aud by their elastic rebound cause the 

 muscle to rapidly resume its relaxed condition when the longi- 



1 Dr. Michael Foster, loc. cit., p. 475, 



