ON THE GENERAL MORPHOLOGY OF THE MYXJNOID FISHES. 711 



says that none of the fibres of the copulo-copularis arise from the inferior chondroidal 

 bar, but in Myxine I have seen it in all my dissections and sections. There is, how- 

 ever, no connection with the superior chondroidal bar. 



It is quite clear from a merely superficial examination that all the fibres of the 

 copulo-copularis, whilst they always pass transversely at right angles to its long axis 

 (except at the posterior extremity), do not form complete half hoops. This is especially 

 well seen in the longitudinal dorso-lateral spindle-shaped projection which extends from 

 one extremity of the muscle to the other. This projection consists of short transverse 

 fibres, the most superficial of which arose from the external fascia 3 mm. from the mid- 

 dorsal line in a 35 cm. Hag, and after a short maximum transverse course of 4 mm. 

 were inserted also into the external fascia (cp. fig. 3). Hence the copulo-copularis is 

 thickest at the region of this projection. In fact by far the greater portion of the 

 surface of the external fascia of the muscle is utilised for the origin and insertion of its 

 fibres, so that in a surface view one sees, apart from the tract above, only the ends of 

 muscle fibres, and not their course, as shown in the above figure. In other words, the 

 muscle fibres become gradually longer as they pass from without inwards. Again, at 

 the anterior region, where the muscle is flattened dorso-ventrally, the fibres are all 

 nearly vertical, passing from the dorsal to the ventral fascia of the muscle, with, however, 

 a slight incline towards the mid-dorsal line. Hence the most internal, and not the most 

 external, fibres are the longest, because they traverse a greater vertical thickness of the 

 muscle, and they, only, pass from the mid-dorsal to the mid-ventral linea. The external 

 fascia may be described as providing the origin and insertion for the greater part of 

 the fibres which course in a slight but internally increasing curve from the dorsal 

 to the ventral fascial regions. At the posterior extremity the fibres do not follow 

 a strictly transverse course, but pass obliquely downwards and backwards, and 

 hence the oblique posterior margin of the muscle is formed by the ends of these 

 muscle fibres. 



By making a median dorsal incision through the posterior fascia and the anterior 

 linea the two halves of the muscle may be pinned out. It is then seen that the muscle 

 forms a hollow tube, complete in front, where it is continuous with the channel of the 

 posterior segment of the basal plate, but only completed behind by the dorsal fascia 

 mentioned above. In this canal is situated the M. longitudinalis linguae behind, and in 

 front its tendon. The cavity is wider behind, where it has to accommodate the belly 

 of the longitudinalis linguae, but gradually narrows in front, where it only has to lodge 

 the tendon. Hence the copulo-copularis is thickest where the channel is narrowest, and 

 it is important to remember this when considering the function of this muscle. Assuming, 

 in the dead state, that the muscle is in contraction, the size of its cavity is subject to 

 enlargement. The median ventral linea is very obvious internally, and the most 

 internal fibres, i.e. those lining the cavity, are seen to be the longest and the most curved, 

 since they arise and are inserted into the most median positions, and hence have to 

 describe a curve in order to enclose a central cavity. 



