1002 



PISCES. 



Fishes is moved by six muscles analogous to 

 those met with in other Vertebrata, and to 

 which similar names are applicable. The recti 

 muscles (jig. 532, 1, 2, 3, 4) are four in 

 number, arising from the back of the orbit 

 near the margin of the optic groove, and run- 

 ning forward to be attached in the usual manner 

 to the sclerotic coat of the eye. The obliqui 

 (fig. 532, a, 6) both take their origin from the 

 anterior part of the walls of the orbit, and pass 

 in a transverse direction towards the eyeball, into 

 which they are inserted, one on its superior, the 

 other on its inferior aspect. There is no troch- 

 lear apparatus in connection with the superior 

 oblique, as is the case in quadrupeds, but, like 

 the inferior, it passes straight to its destination. 

 The suspensory or choanoid muscle met with 

 in Mammalia, in Fishes is totally wanting. 



In the Sharks the muscles moving the eye- 

 ball are of very great strength, and, moreover, 

 their efficiency is rendered more perfect by me- 

 chanical contrivances that are not met with in 

 the ordinary Fishes. In the latter the eye is 

 simply supported in the orbit by a quantity of 

 loose cellulosity filled with a gelatinous or fatty 

 semifluid substance, admirably adapted to faci- 

 litate the movements of the eye; but in the 

 plagiostome cartilaginous Fishes the cartilagi- 

 nous pedicle is provided, already mentioned, 

 which, taking its origin from the back of the 

 orbit between the origins of the recti muscles, 

 runs forward to be moveably articulated, fre- 

 quently by means of a very complete ball- 

 and-socket joint enclosed in a capsular liga- 

 ment, to the back of the selerotic, so as to 

 form a pivot upon which the eye turns. In 

 the attachment of the recti and oblique mus- 

 cles to the eye-ball an additional piece of me- 

 chanism is observable, each of these muscles 

 being inserted into a prominent cartilaginous 

 tubercle, which projects from the external sur- 

 face of the sclerotic, and thus enables the 

 muscle to act with greater advantage. 



In the generality of Fishes there are no eye- 

 lids, the external tegument passing on to the 

 front of the eye-ball without forming any fold 

 or duplicature to which such a title is appli- 

 cable; there are, however, exceptions to this 

 arrangement which must not be passed over 

 unnoticed. Thus, in the Mackarel (Scomber 

 Scombrus), the eye is partially defended by two 

 vertical folds of the common integument, and 

 in the Herring ( Clupea Harengus) there is a 

 similar provision for the defence of the eye-ball 

 and orbit.* The vertical folds are unprovided 

 with any muscular structure for their move- 

 ment, and are consequently transparent so as 

 not to interfere with vision when the front of 

 the eye is brought beneath them. It is worthy 

 of observation that, where these folds decussate 

 one another at their inferior extremities, the an- 

 terior one overlaps the posterior, so slight an 

 impediment to progressive motion as the con- 

 trary position would have occasioned having 

 thus been foreseen and avoided. 



In the Sharks and Sturgeons the integument 



* Vide Catalogue, Mus. Coll. Surgeons, Lond. 

 vol. iii. p. 171. 



forms a deep circular fold around the front of 

 the eye, which, although motionless, is evidently 

 of a palpebral character. A secreting membrana 

 conjunctiva is reflected deeply between this cir- 

 cular fold and the globe of the eye, of which it 

 covers the anterior half. In the Sharks* there 

 is likewise a third eyelid, which is moveable; 

 this is placed at the inferior and internal or 

 nasal side of the orbit, and is moved over the 

 front of the eye in a direction upwards and out- 

 wards by means of a strong round muscle (nic- 

 titator) which arises from the upper and poste- 

 rior or temporal side of the orbit, and descends 

 obliquely to be inserted into the lower and 

 outer margin of the third eye-lid; passing in 

 this course first through a muscular trochlea, 

 and then through a ligamento-cartilaginous 

 loop. The trochlear muscle is not, however, 

 exclusively subservient to the action of the 

 nictitator, but has an insertion in the upper 

 part of the palpebral fold, which it depresses 

 simultaneously with the raising of the third 

 eyelid, a slight external groove above the upper 

 eyelid indicating the extent of motion allowed. 

 The lacrymal apparatus is totally wanting in 

 the whole race of Fises, no trace of lacrymal 

 glands or punctae lacrymalias being ever distin- 

 guishable ; neither could a lacrymal secretion 

 be needed in animals whose eyes are perpetu- 

 ally bathed by the water in which they live. 



Auditory apparatus. The organ of hearing 

 in Fishes undergoes a gradual improvement in 

 its structure as we advance from the lower to the 

 more highly organized genera, presenting almost 

 every intermediate gradation between the least 

 complex form, in which it consists of the vesti- 

 bule alone, without semicircular canals or other 

 appendages, approximating in simplicity the 

 ear of a Cuttle-fish (vide art. CEPHALOPODA), 

 to the most complete icthyic type of the 

 auditory apparatus, met with in the Sharks and 

 Sturgeons. 



It is in the Lampreys (Petrorm/zon) that 

 the auditory organ exists in its humblest state 

 of developemenl.f In these Fishes the ear is 

 enclosed in a simple cartilaginous capsule of 

 an elliptical figure, situated on each side of the 

 skull external to the posterior cranial cartilages. 

 The walls of these capsules are thin, and the 

 cavity which they contain of an ovoid shape. 

 In that side of each cartilaginous capsule 

 (vestibulum cartilagineum, Weber,) which is 

 nearest the cranium, are two openings, the 

 inferior, which is the larger, being of an oval 

 shape closed with a firm and elastic membrane, 

 while the superior is extremely small, giving 

 transit to the auditory nerve as it passes into 

 the vestibule. With the exception of these 

 apertures, which open into the cavity of the 

 cranium, the cartilaginous capsule is closed on 

 all sides. 



The whole of the elliptical cavity of the 

 cartilaginous capsule is filled by a pellucid 

 membranous sac (vestibulum mcmbranaceum ) 



* Catalogue, Mus. Coll. Surgeons. Lond. prep. 

 1762. 



f Vide Tract, de Aure animalium aquatilium, 

 auctore Ernesto Henrico Webero. Lipsiae, 1820. 

 4to. 



