:m 



ANATOMY OF VERTEBRATES. 



219 



of the globe. In Orthagoriscus 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 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 simul- 

 taneously 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, fig. 219, i,2,3,4,andtwo 

 obliqui, ib. a, b : the latter rise from the nasal 

 side of the orbit, and are inserted most favour- 

 ably for effecting the rotatory movements of 

 the eyeball : but the superior oblique, a, 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 carti- 

 laginous sclerotic for the common insertion of 

 the rectus superior and obliquus superior ; 

 and a second protuberance below for the common insertion of 

 the obliquus inferior and rectus inferior. The recti muscles 

 rise in many Osseous Fishes from the sub-cranial canal ; 2 

 the origin of the rectus externus 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 



Coats of the eye of the Pereh 



XXIII. 



1 The family of Sharks, including Galeus, Carcharias, with this grade of palpehral 

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

 of these great predatory Fishes. 



2 If, therefore, we regard this canal as part of the orbits, we must add the alisphenoid, 

 basisphenoid, and even the basioccipital, to the bones enumerated at p. 1 1 G, as forming 

 the chambers for the eyeballs and their appendages in Fishes; and this multiplicity of 

 orbital bones interestingly repeats or parallels the characteristic formation of the 

 otocranes or ear-chambers in the present class. 



