732 MR FRANK J. COLE 



only backwards, none of the fibres passing forwards as above described. I have seen 

 this condition also, but only in one specimen. 



Ventral Longitudinal Tract. — The fibres of the two tracts, when traced backwards 

 are seen to gradually converge as they approach the external branchial openings. Most 

 of their fibres arise immediately behind these openings — the fibres of the two sides 

 meeting at the mid-ventral line, where they are attached to the cardiac portion of the 

 constrictor. The tract also receives dorso-ventral fibres which bend round on to it 

 from above behind the ductus cesophago-cutaneus, as described below, and also a few 

 fibres from the dorsal longitudinal tract, which pass round in a curve into it behind the 

 above ductus instead of terminating at the mid-ventral line, as described below. The 

 only traces I have seen of any decussation in the constrictor is that a variable number 

 of fibres of, say, the right dorsal longitudinal tract may cross over into the left ventral 

 longitudinal tract, and vice versa. 



We must now enter into the details of the anatomy of the posterior region of the 

 constrictor — a subject both difficult to understand and to describe. To get at the facts 

 it is necessary to remove the constrictor en bloc and carefully clean it up. This is not 

 an easy dissection, but it is the only way to understand the muscle. To do it, you 

 remove the whole of the branchial region from the body-wall, leaving the rectus muscle 

 attached to the latter. Then remove first the efferent gill ducts and then the gills 

 themselves on each side, and finally snip through the large loop on the left side in 

 front of the ductus cesophago-cutaneus, so that the latter itself may be cleared away. 

 The constrictor is now ready to be isolated. Make a longitudinal incision through the 

 thickness of the muscle a little on one side of the mid- dorsal line, and with a fine 

 needle separate the constrictor from the gut, which may then be lifted out. You now 

 have the whole of the constrictor in one piece, so that both inner and outer surfaces 

 may be investigated, and further examination effected under the higher powers of the 

 dissecting microscope. 



The cardiac portion of the constrictor (figs. 2, 3, 13, c.b.c".) extends a variable 

 distance behind the gill region over the gut — in an average specimen about 5 mm. In 

 one case the fibres were separated at the mid-dorsal and mid-ventral lines by a 

 ligamentous septum. The fibres are loosely arranged behind, but are more compacted 

 in front. They course transversely round the gut, but usually slightly obliquely 

 forwards from above. Dorsally and in front the fibres surround the exit of the 

 systemic aorta so that the latter vessel has a small constrictor of its own. In front 

 of this region dorsally and on each side the fibres again diverge to allow 

 the exit on the left side of oesophageal vessels opening into the left anterior cardinal 

 (figs. 2, 3, 13, 12), and on the right side of similar vessels opening into the anterior 

 portal vein, which is a smaller vessel, but occupies a similar position to the anterior 

 cardinal of the other side. These openings may be provided with definite constrictor 

 fibres. The mere divergence of the fibres could of course perform the same function, 

 but not so effectively as a special constrictor. Ventrally in front the cardiac portion of 



