.336 



ANATOMY OP VERTEBRATES. 



219 



of the globe. In Ortlia.fjoriscus the circular palpebral fold is 

 deeper, and is provided with a sphincter : in most Scomberoid and 

 Clupeoid Fishes there is an anterior and a posterior vertical trans- 

 parent fold or eyelid. In the eye of the tope and blue Shark, 

 there is a nictitating membrane superadded to a well-developed 

 circular palpebral fold of the skin. A conjunctive membrane is 

 reflected from the circular eyelid 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 ' mus- 

 cle rises from the temjwral 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 siuuil- 

 taneously with the raising of the third lid.' The 

 jiroper muscles of the eyeball exist in all fishes 

 except the Myxinoids and Lepidosireu, and 

 consist of the four recti, fig. 219,1,2,3,4, and two 

 oliliqui, ib. «, h : the latter rise from the nasal 

 side of the orbit, and are inserted most favour- 

 ably for eflPecting the rotatory movements of 

 the eyeball : but the superior oblique, a, has 

 not its direction changed by a trochlea in the 

 2)rcsent class. In the Gahnis there is a special 

 ])rotuberance of the upper part of the carti- 

 laginous sclerotic for the common insertion of 

 the rectus siiperior and obliqiD/s superior ; 

 and a second protuberance below for the c<:iniinon insertion of 

 the ohliquus infer/or and rectus inferior. The recti muscles 

 rise in many Osseous Fishes from the sub-cranial canal ; '^ 

 the origin of the rectus e.rtermis being prolonged furthest liack. 

 But the recti muscles are most remarkable for their length 

 in the Hammer-headed Sharks, since they rise from the basis 

 cranii, and extend ak)ng the lateral processes or peduncles, at the 



cil lliCCyoof the Tel 

 XXIIJ. 



' The family of Sharks, inclmliiig Gulciis, Carcharias, with this grade of ]ialpcbral 

 structure, arc called ' nictitantcs ;' thej' are amongst the most active and formidable 

 of these great predatory Irishes. 



- If, therefore, wo regard this canal as part of the orliits, we must add the alisphciioid. 

 basisphenoid, and even the basioccipital, to the bones eimnieratcd at p. 1 1 6, as forming 

 the chambers for the eyeballs and their appendages in Pishes; and this nuiltiplieity of 

 orbital bones interestingly ic)icats or jiarallels the characteristic formation of the 

 otocranes or car-chambers in the ja'cscnt class. 



