Noa3:|) JZUSCLES AND NERVES IN AMTA CALVA. 531 
the rectus internus, and not from the retractor bulbi, as the 
Sarasins suggest. Ichthyophis, therefore, presents a condition 
intermediate between Urodela and Anura, and may represent 
the beginning of the line leading from Amphibia to Sauropsida. 
Burckhardt (No. 16) also places Ichthyophis intermediate 
between Anura and Urodela, but derives it from Protopterus, 
the Sauropsida being derived from Ceratodus and representing 
a totally different line. In a later work I believe he has stated 
that it represents, in the arrangement of the different parts of 
the brain, a type leading directly to Sauropsida and the higher 
vertebrates. I do not, however, find the reference. 
In the higher vertebrates the retractor bulbi may persist 
or it may disappear, probably by fusion with the rectus externus, 
as indicated by the occasional double muscle in man, and also 
by its innervation in man, the abducens entering the orbit 
between the two heads of the muscle (No. 100, vol. II, pp. 290 
and 291). 
In this ancestral tree of the muscles and nerves of the 
eyeball the several lines of descent are sharply and positively 
defined, but they are based on most insufficient and perhaps 
inaccurate data. The important assertion that reversions have 
not occurred must also be granted. If that be allowed, the 
other assumptions, or rather deductions, are not important or 
improbable, and the schema becomes an ancestral tree of ver- 
tebrates. In this tree it will be noticed that the lines leading 
to the higher types of each class resemble each other in that 
the superior branch of the oculomotorius in such lines inner- 
vates but one of the muscles of the eye; that in the lines 
leading to the lower types it always innervates two ; and that 
the line leading from one class to the next higher arises be- 
tween the lines leading to the two types of the class ; that is, 
from the so-called proto-urodele type upward there has been 
in the development of the muscles of the eyeball but one 
impulse, if it may be so called, leading to the formation of 
the arrangements found in higher types, and not repeated ones. 
The forms indicated in the schema as intermediate ones are 
the Holocephala, Polypterus, Ichthyophis, and the Monotre- 
mata, and the grouping of the several orders is exactly the same 
