ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 685 



the external surface, i.e. the surface abutting on the subcutaneous blood sinus, consists 

 of red fibres two or three rows deep (apparently observed by Schneider), and from this 

 crust there penetrate inwards, often right through the thickness of the muscle, numerous 

 horizontal sheets of red fibres, each of which is shown in transverse section to be formed 

 of a single row of fibres placed end to end, like a row of bricks. It is curious that, as 

 already pointed out by Maurer, the red fibres are confined to the ventral surface of 

 the fasciculi, the remainder of each fasciculus (with its dorsal surface) being formed 

 entirely of the white fibres. Thus every fasciculus has a ventral sheath of red fibres, 

 in the Lamprey, according to Maurer, the red fibres extend right round the fasciculi. 

 The following descriptions are based on fibres selected from the same specimen, and 

 take no account of the state of contraction in which they happened to be. 



M. parietalis. 



White Fibres (fig. 7, a). — These are of large size and are formed of small sarco- 

 styles, (-?),* which are very numerous and closely packed. The transverse striation is 

 sharply marked, and there is no peripheral zone of sarcoplasm nor a peripheral blood 

 plexus. The nuclei (%) are numerous, narrow, very elongated, and mostly internal, 

 although some are situated superficially (all seen in transverse section in the figure). 

 According to Maurer, the distorted appearance which these nuclei often present is due 

 to the pressure of the surrounding sarcostyles. They are also said to resemble the 

 sarcolemma nuclei. 



Red Fibres (fig. 7, b). — These are of much smaller size and have slightly larger 

 sarcostyles (1) which are not so concentrated (i.e. not so numerous). According to 

 Maurer, the sarcostyles are here aggregations of smaller fibrillse, but I find no evidence 

 of this. Sarcoplasm (with an occasional stray nucleus — S) is quite visible as a packing 

 between the sarcostyles. According to Maurer, however, there are no internal nuclei 

 in the red fibres ; and whilst this is not literally true, it is correct to say that internal 

 nuclei are typically absent in the red fibres. The transverse striation is not so well 

 marked as in the white fibres, but the longitudinal striation is very apparent. Peri- 

 pherally there is a zone of sarcoplasm (3) which lodges the large oval or circular true 

 nuclei (4) of the fibre. The nuclei are here, therefore, not central, as in the white fibre, 

 but peripheral. Round each fibre there is a rich peripheral blood plexus (5) which is 

 very striking in longitudinal sections stained with methyl-blue-eosin. It is obvious 

 that these are rapid fibres undergoing considerable tissue change, and hence the zone of 

 nutritive sarcoplasm with its extensive vascular supply. 



M. cordis caudalis. (Fig. 7, c.) 

 This muscle consists of red fibres only. These on an average are much smaller than 

 the similar fibres of the parietalis, but they vary considerably in size. The figure has 



* Perhaps these should be regarded rather as fibrils which have not collected together to form clearly denned 

 muscle columns or sarcostyles. 



