234 - ALUS. [Vol. XVIII. 



facial opening, and becomes the jugular vein. The other two ves- 

 sels run backward ventral to the obliquus superior, but dorsal to 

 the obliquus inferior, and then backward and downward mesial 

 to the rectus internus. Having reached the lower edge of the lat- 

 ter muscle they turn upward and backward around that edge and 

 around the corresponding edge of the rectus inferior, and pass- 

 ing antero-mesial and then dorsal to the rectus externus enter the 

 eye-muscle canal by its orbital opening. Their further course 

 could not be traced. They accompany, in their course through 

 the orbit, the inferior branch of the nervus oculomotorius, and they 

 are joined, before entering the eye-muscle canal, by a large vein 

 that comes from the eyeball, the three veins apparently uniting to 

 form a single vessel. The venous trunk thus formed, and its 

 anterior continuation in the vein that comes from the eyeball, thus 

 together correspond in position to the vein ov of my descriptions 

 of Amia. In Scomber no communicating branch to the more dorsal 

 orbital vein could be established, nor could it be determined 

 whether or not the ventral vessel communicated .with the corre- 

 sponding vessel of the opposite side of the head, and supplied the 

 hypophysis or saccus vasculosus, as in Amia. 



3. Nervus Opticus. 



The Nervus Opticus (0), on each side, issues from the cranial 

 cavity antero-dorsal to the anterior edge of the basisphenoid, and, 

 lying postero-ventral to the eye-stalk, runs forward and laterally 

 to the eyeball. In issuing from the cranial cavity it pierces the 

 interorbital membranes at the point where the unpaired interorbital 

 septum spreads, on each side, to form or join the membrane that 

 closes the orbital opening of the brain case. It is a large, strong 

 nerve, enclosed in a tough cylindrical envelope, but the nervous 

 matter itself is arranged in many folds inside the envelope. Stan- 

 nius says that the nerve is found in the form of a folded band. 

 The two nerves cross each other while under the ventral surface 

 of the brain, sometimes one and sometimes the other having the 

 ventral position. Whether there was here a commissure connecting 

 the nerves, or any interchange of fibers, was not investigated. 

 Stannius says there is a simple crossing of the nerves. 



