736 MR FRANK J. COLE 



and upwards into the dorsal longitudinal tract, and some I have traced to an origin 

 on the ventral surface of the cardiac portion of the constrictor. 



On the right side, owing to the absence of the ductus oesophago-cutaneus, the ventral 

 insertion is naturally the smaller by the fibres which are contributed to it on the left 

 side by the fibres of the ductus. Hence the outer portion of the insertion is formed 

 practically entirely by the last or sixth loop. The inner portion resembles that of the 

 other side. 



The physiology of this complex muscular apparatus can, of course, only be deter- 

 mined by experiment. Two points, however, are quite clear at once : (1) the oeso- 

 phagus can be powerfully constricted immediately behind the branchial region so as to 

 prevent the entrance of respiratory water into the gut ; (2) the ductus oesophago- 

 cutaneus, lying as it does for the most part in a strong muscular tube, as well as 

 possessing an abundant intrinsic musculature, can also be occluded. For the rest, the 

 muscle is subject to so much variation as to indicate that it is a vestigial structure of 

 little, if any, physiological importance. It certainly cannot in the average Myxine 

 "discharge the water taken in by the breathing organs" (J. Muller, p. 276). For 

 this the powerful intrinsic musculature of the gills and their ducts is amply sufficient. 

 It is quite possible, however, that it may be concerned in controlling the blood-pressure 

 in the elaborate system of vascular spaces surrounding the gill region. Muller's 

 statement of the physiology of the gill apparatus is evidently based merely on its 

 anatomy, and he further does not seem to be aware that the respiratory water normally 

 reaches the gills through the nasal tube. 



I append the following notes in explanation of the reconstruction of the general 

 anatomy of the left side of the constrictor muscle from my large series of sections 

 (fig. 2). I do not here or elsewhere include the intrinsic musculature of the gut and 

 gill apparatus, which will be described in my next Part when these structures are 

 under special consideration. 



The posterior external sheet (c. b. a.) has a very ragged ventral border, and, judging 

 from the structure of the loops as we pass backwards, is evidently formed by the 

 fusion of those fibres of the last three loops which are related to the gills, i.e. the non- 

 ventral fibres. These become more numerous and diffuse from before backwards, and 

 finally fuse up to cover the last gill and the greater portion of the fifth. There are 

 numerous lacunae here and elsewhere (not all shown in the Chart), the largest (12) 

 transmitting . the veins from the gut opening into the anterior cardinal on the left 

 side and the anterior portal on the right. The lacuna is smaller on the right side. 

 The two halves of the constrictor fuse dorsally at about 2970, but in front of this they 

 are connected by three transverse bridges — a narrow anterior and posterior one, and a 

 wide intermediate one. The constrictor envelops the systemic aorta (s. ao.) at the 

 mid-dorsal line as above described. In front the muscle forks in the usual way into 

 the pair of anterior limbs (c. b. c'.), and each of these passes, without any obvious line of 



