522 ALLIS. [Vou. XII. 
These different relations of the oculomotorius to the internal 
and superior recti, in elasmobranchs, are due to and are caused 
by the gradual shifting from before backward of the origins of 
all the recti muscles, and also of the place of exit of the oculo- 
motorius from the cranium. As a result of this shifting the 
internal and superior recti, at their origins, either traverse, or are 
traversed by, the issuing nerve. The nerve does not traverse the 
inferior rectus also, by which it may, as in Galeus canis (Cut 2), 
be pulled backward and much diverted from its course, simply 
because, crossing the muscle as it does midway in its length, 
such a process is, or at least would seem to be, impossible 
without destroying or disturbing functionally the muscle. 

Cut 2.— Same as Cut 1 with rectus superior and obliquus superior muscles removed. ocw2, oculo- 
motorius; ocm?, inferior branch of oculomotorius ; 044, ophthalmicus profundus trigemini; rz/, 
rectus inferior. 
In Petromyzon (No. 37, pp. 38, 58, and 70) the trochlearis 
innervates the superior oblique, which arises from the mem- 
branous hind wall of the orbit, and the abducens innervates 
the inferior as well as the external rectus. The trochlearis, as 
it issues from the cranium, lies dorsal to the abducens and 
apparently dorsal also to the main trunk of the trigeminus, for 
Fiirbringer, speaking of the roots of the three nerves and their 
relations to each other as they issue from the cranium, says of 
the trochlearis, that it is ‘‘der am oberflachlichsten gelegene.” 
If such be its position it must, as it issues from the cranium, 
run backward and outward above and across the other two 
nerves. It must therefore lie dorsal to the r. ophthalmicus 
trigemini, a position confirmed by Ahlborn (No. 1, p. 21). This 
ophthalmic nerve in Petromyzon does not, therefore, probably 
