1890.] 17 [Ryder. 



The eye and gland together measured 2.5 mm. in the longest diameter, 

 and about 1.5 mm. thick, and nearly 2 mm. wide. The form of the whole 

 mass was that of a depressed oval. So far as the evidence goes, that can 

 be derived from the sections, it indicates that the ocular muscles do not 

 reach the skull, and that the eyes are no longer under the control of the 

 same kind of a muscular mechanism as is found in other mammals. In 

 fact, the tendons and muscles of the snout seem to completely cover the 

 skull in the region of the orbit. Indeed so slight is the attachment of the 

 eyes to the skull, that in removing the skin from the head but little diffi- 

 culty is found in removing the eye-ball and lachrymal gland with the 

 former. In Scalops, our common mole, this happens with still less dif- 

 ficult y. 



The whole eye-ball in Blarina measures 0.9 mm. in diameter or consider- 

 ably less than one-twenly-fifih of an inch. The lens is well developed and is 

 very large in proportion to the whole eye-ball, measuring more than half the 

 diameter of the latter. Consequently there i? but little aqueous humor, 

 and also but little vitreous, since the lens fills nearly the whole of the 

 chamber of the ball. The cornea is thin, very convex, and approximates 

 the lens anteriorly. At the edge of the cornea there is no thickening of 

 the sclerotic, such as occurs in the eyes of other vertebrates as a result of 

 the development of the ciliary muscles or apparatus of accommodation. 

 This apparatus is obviously very rudimentary and defective, from which 

 it may be inferred that the power of adjustment of the lens for different 

 ranges of vision is poorly developed in Blarina. 



There is a retinal coat of pigment and a choroid coat, which latter 

 extends for a little distance over the optic nerve. The thickness of the 

 sclerotic, choroid and retinal layer of pigment taken together is not over 

 a fourth of the total thickness of the retina, thus showing other strong con- 

 trasts in respect to the development of the tunics of the eye-ball in other 

 forms of vertebrates. 



The total thickness of the retina is nearly a third of the total diameter of 

 the eye-ball, and is developed as far forward as the ciliary region, though 

 it is thickest a, little behind this point. The usual number of layers are 

 discernible in the retina, and it is perforated as usual by the very slender 

 and diminutive optic nerve, which is only .07 mm. in diameter. The 

 retina is therefore developed as usual, though the rods and cones were not 

 well enough preserved in my materials to be certainly made out. At any 

 rate, it is clear that such an eye may still be more or less functional as a 

 visual organ even though obviously degenerate in some respects. The 

 number of retinal elements is absolutely and relatively much less, how- 

 ever, than in a larger -eye where the arc covered by the retina is part of a 

 larger circle than in Blarina. There is a well-defined iris and pupil. 



The lachrymal gland is many times larger than the entire eye ball. Its 

 duct opens into the conjunctival cavity. 



The muscles of the eye consist, first, of a choanoid muscle or retractor 

 of the ball. It is inserted upon the sclerotic in a circular manner near 



PROC. AMER. PHILOS. SOC. XXVIII. 132. C. PRINTED FEB. 25, 1890. 



