118 STUDIES IN ADVANCED PHYSIOLOGY. 



cylinder expand into a broad plate-like structure known as 

 the muscle plate, or the nerve plate. In this nerve plate 

 occur numerous nuclei. This nerve plate is probably noth- 

 ing more than the expansion of nervous protoplasm by 

 which the contact between nerve and muscle is made the 

 more intimate. 



Color. Ordinarily voluntary muscles appear red. Ex- 

 ceptions to this are, however, familiar to us all in the case 

 of the breast meat of domestic birds, which is white. 

 The redness of ordinary muscle is due to its containing 

 traces of haemoglobin, which it has probably derived from 

 the continued coursing of the blood in such large quantities 

 through its substance. Muscles which are not much used 

 and which, therefore, have their blood supply much re- 

 duced, are white. Thus in wild birds which use their 

 wings in continued flight, the breast muscles also are dark. 

 Whether this haemoglobin is contained in the muscle for a 

 specific purpose, or as a mere accidental coloring, caused 

 by the circulating blood, is not clear. Possibly it may 

 enable the muscles to store up in themselves for ready use, 

 a small reserve supply of oxygen. 



Growth of Muscle. The dimensions of a muscle just 

 given are by no means invariable. In fact, the individual 

 muscle fibres actually grow in size for a long time, being 

 many times larger in an adult than in a very young child. 

 The growth of muscle, resulting from continued properly di- 

 rected muscular exercise, such for instance as with the 

 blacksmith in the use of his arms, increases the size, and so 

 of course, strength of the individual muscle fibres already 

 formed. Reasoning in an opposite direction, the apparent 

 atrophy of. an unused muscle may be due to a peculiar 

 shrinking of the individual muscle fibres rather than a 

 direct absorption of them. But it seems probable that new 

 muscle fibres may also arise after birth. There are found 

 scattered throughout the muscular substance small spindle- 

 shaped cells, designated as muscle cells, or more frequently 



