206 LECTURE vni. 



the skin. A conjunctive membrane is reflected fromtlie circular eye- 

 lid over the third eyelid, which is placed at the nasal side of the orbit, 

 and then passes over the anterior half of the eyeball. A strong 

 ' nictitator' muscle rises from the temporal side of the orbit, and passing 

 through a muscular and ligamentous loop, descends obliquely to be 

 inserted into the lower margin of the third lid. The trochlear 

 muscle has an insertion into the upper part of the circular lid, and 

 depresses that part simultaneously with the raising of the third 

 lid. * 



The proper muscles of the eyeball exist in all fishes except 

 the Myxinoids and Lepidosiren, and consist of the four recti 

 and two obliqui : the latter rise from the nasal side of the 

 orbit, and are inserted most favourably for effecting the rotatory 

 movements of the eyeball : but the superior oblique has not its 

 direction changed by a trochlea in the present class. In the Galeus 

 there is a special protuberance of the upper part of the cartilaginous 

 sclerotic for the common insertion of the rectus superior and obliquus 

 superior ; and a second protuberance below for the common insei'tion 

 of the obliquus inferior and rectus inferior. The recti muscles rise 

 in many Osseous Fishes from the sub-cranial canal f ; the origin of the 

 rectus ex^er/^^^* being prolonged furthest back. But the recti muscles 

 are most remarkable for their length in the Hammer-headed Sharks, 

 since they rise from the basis cranii, and extend along the lateral 

 processes or peduncles, at the free extremities of which the eyeballs 

 are situated. In all Plagiostomes the eyeball is supported on a car- 

 tilaginous peduncle : this is short and broad in the Rays ; longer and 

 cylindrical in the Sharks ; in the Selache it is articulated by a ball and 

 socket synovial joint to a tubercle above and external to the entry of 

 the optic nerve. A fibrous ligament attaches the sclerotic to the 

 wall of the orbit in the Sturgeon and the Salmon. 



The space between the eyeball and the orbit contains a soft bed of 

 gelatinous and adipose substance : but there is no lachrymal gland in 

 fishes. An apparatus to moisten the cornea was, of course, unnecessary 

 in animals perpetually moving in a liquid medium. The cornea, which 

 in most fishes is always exposed to that medium, is flat ; it is, therefore, 

 less liable to injury in the rapid movements of the fish, and being 



* Prof. Miiller has established the family ' Nictitantes ' for the Sharks, includino- 

 the Galeus, Carcharias, and a iew other genera, with the third eyelid. 



t If, therefore, we regard this canal as part of the orbits, we must add the ali- 

 sphenoid, basi-sphenoid, and even the basi-occipital to the bones enumerated at 

 p. 103., as forming the chambers for the eyeballs and their appendages in Fishes; 

 and this multiplicity of orbital bones interestingly repeats or pai-allels the charac- 

 teristic formation of the otocranes or ear-chambers in the present class. 



