PISCES. 



973 



the fish, as may easily be proved by drying it 

 with a napkin, after which operation it soon be- 

 comes again covered with mucus, which issues 

 from the openings of these pores. 



In the Tunny (Scomber thynnus) there runs 

 beneath the skin, following the entire length of 

 the lateral line, an organ of a redder colour 

 than the rest of the flesh, from which the little 

 tubes forming the lateral line proceed, each 

 tube receiving a nervous filament from the 

 great lateral nerve. On raising the integument 

 over this glandular organ a large vessel is seen, 

 which, besides giving off arteries to the neigh- 

 bouring muscles, furnishes an infinite number of 

 branches to the glandular mass, beneath which, 

 at nearly an inch from the surface, runs the lateral 

 branch of the eighth pair of nerves, which in 

 most other Fishes is situated immediately be- 

 neath the skin. It is in the Raidse or Skates, 

 however, that this system of vessels is most 

 largely developed. In these broad-bodied fishes 

 there is found upon the ventral aspect of the 

 body a large canal which surrounds the pro- 

 minent muzzle, forming very regular angles and 

 windings, distributes its secretion by three or 

 four branches on each side, and then winds 

 upwards to terminate by different openings. 

 There is, moreover, on each side at the external 

 angle of the branchiae a kind of sac which is 

 round and of a whitish colour, which receives a 

 large branch from the fifth pair of nerves, from 

 which proceed a number of long simple vessels 

 which run in radiating fasciculi in four or five 

 different directions, and open at remote points 

 on the surface of the body. 



In Sharks the entire substance of the snout 

 is made up of a dense cellulosity filled with a 

 mucilaginous fluid, in which are imbedded fas- 

 ciculi of tubes that open upon the surface of the 

 skin by wide orifices. Besides these there are 

 large vessels of similar character, one of which 

 runs along the whole length of the animal on 

 each sid-e. Innumerable muciparous follicles 

 contribute likewise to lubricate the skin, more 

 especially in the vicinity of the snout. 



By far the greater number of genera in the 

 class before us are covered with imbricated 

 scales, which overlap each other like the tiles 

 of a house ; the external and visible portion of 

 these scales is covered with a thin layer of der- 

 mis, which soon dries on exposure to the air; 

 their internal or concealed part is lodged in a 

 cavity which is a kind of sacculus hollowed out 

 in the dermis itself, or formed by one of its 

 replications an arrangement which at first 

 sight appears very different from what exists in 

 Lizards and Serpents, in which what is called 

 a scale is only a production of the cutis covered 

 by the epidermis, that on the outer surface 

 assumes a greater consistency and thickness ; 

 but in the genus Scienus we have an inter- 

 mediate arrangement between the imbricated 

 scales of Fishes and what is met with in the 

 scaly Reptilia. In the genus above mentioned 

 the folds of the dermis are occupied by a cal- 

 careous plate, constituting a true scale easily 

 separable from the cutis which envelopes it. 

 We have only therefore to suppose the texture 

 of this layer of cutis to be thinner and more 



delicate, and we arrive at once at the scale of 

 a fish, which seems in a fossa excavated in the 

 cutis. In Fishes the scales thus implanted in 

 the true skin were supposed by Cuvier to have 

 no vascular connection with it, but to originate 

 like a shell in the mantle of a mollusk by the 

 gradual deposition of consecutive layers depo- 

 sited from the dermis ; and all their varieties of 

 surface, their different sculpture, the ridges or 

 spines with which they are sometimes armed, 

 and which frequently render them very beautiful 

 objects for the microscope, were generally 

 thought to have a similar origin. 



Dr. Mandl* appears to have been the first 

 who, by a microscopic examination of the inti- 

 mate structure of the tissues which enter into 

 the composition of the scales of Fishes, arrived 

 at just conclusions relative to the mode of their 

 formation, and proved that, so far from being 

 mere exudations of corneous matter, they are 

 produced, like the teeth and osseous tissue, by 

 a true internal growth and nutrition. 



The following is an abstract of the result of 

 Dr.Mandl's researches upon this interesting sub- 

 ject, in which he satisfactorily proves that the 

 scales of Fishes consist of two layers, of which 

 the inferior exhibits a structure analogous to 

 that of fibro-cartilage, whilst the superior re- 

 sembles corpuscular cartilage, and is evidently 

 formed by the developement of primitive cells. 



Taking a well-developed scale, as that of a 

 Carp, for an example, it is easy to perceive that 

 its surface is marked with longitudinal lines 

 arising from a common centre, and running to- 

 wards the periphery of the scale, the number 

 of which it is generally very easy to determine. 

 The place towards which these lines converge 

 is a space of variable dimensions, called by 

 Dr. Mandl the focus. Between the longitudinal 

 lines are seen, running parallel to the circum- 

 ference of the scale, a very considerable number 

 of concentric lines, which are crossed by the 

 longitudinal ones at right angles ; these are 

 named " cellular lines" because they owe their 

 origin to the developement of cells. Besides 

 the parts above mentioned, many kinds of 

 scales exhibit upon their surface, and upon 

 one of their edges, spines of different forms, 

 called by Dr. Mandl the teeth of the scale, a 

 name which he founds upon the mode of de- 

 velopement of these appendages. Around the 

 longitudinal and transverse lines, more espe- 

 cially near the point where the former converge 

 towards the " focus," are numerous yellowish 

 corpuscles of an elliptical shape, named the cor- 

 puscles of the scale. 



Lastly, if the upper layer of the scale be 

 raised or torn, an inferior stratum is displayed, 

 of a fibrous character. These different struc- 

 tures he then proceeds to describe seriatim. 



1 . The longitudinal lines, which, arising 

 from the focus of the scale, run towards its 

 periphery, play an important part in the ana- 

 tomy of the tissue we are examining, and when 

 highly magnified are found to be so many ca- 

 nals exhibiting in the scales of different species 



* Recherches sur la Structure interne des Ecailles 

 des Poissons. Par le Dr. L. Mandl. Ann. des 

 Sc. Nat. torn. xi. 



