TEGUMENTRAY ORGANS. 



501 



areolar tissue, and receive twigs from the 

 vessels in their neighbourhood. 



In the other Mammalia, the general structure 

 of the sudoriparous glands is as in man. In 

 the sheep, according to Gurlt.they present the 

 same coiled arrangement, while in the ox and 

 dog they are straight and simple. In the ox 

 they have rounded, d.lated extremities, and 

 are everywhere similar in shape and size. On 

 the hairy parts of the body of the dog, they 

 are small simple coeca, which are very difficult 

 to discover ; while on the ball of the foot of 

 this animal they are very large and resemble 

 those of man. Very large sudoriparous glands 

 have likewise been observed upon the horse's 

 prepuce. 



Scales of fishes. In the Ganoid fishes Ac- 

 cipenser and Polypterus the substance of the 

 scales is composed of ordinary bone whose 

 superficial layer is only denser than the rest, 

 and exhibits a local developement of fine 

 branching tubuli j but in other fishes, two, if 

 not three, distinct layers are usually distin- 

 guishable in the scales. 



In many Plagiostomes, for instance, the 

 placoid scales have the same composition as 

 the teeth, consisting of a superficial layer of 

 nearly structureless dense " enamel," or as 

 Prof. Williamson more conveniently terms 

 it, " Ganoin," while the deeper substance is 

 composed of a tissue in every respect similar 

 to dentine, whose innermost portion in some 

 cases passes into true bone, an addition 

 which might be compared to that of the 

 cement in the teeth. Leydig, indeed, has 

 shown that the resemblances between the 

 scales and the teeth of Placoid fish extend 

 even to their mode of developement. If the 

 pulp contained in the central cavity of the 

 spine-like scale of a Raia clavata be pulled 

 out, globular calcareous masses of T oW of an 

 inch and upwards in diameter, and either 

 solitary or adhering together in masses, will 

 be found to be attached to its surface. " These 

 globules are exactly analogous to the dentine 

 globules described by Czermak, which in 

 human teeth afford the formative material for 

 the matrix of the dentine. What, however, ap- 

 peared to me especially worthy of notice 

 was the circumstance, that the most distinct 

 and beautifully branched canals, having exactly 

 the same appearance as those in the substance 

 of the spine, were already visible in these 

 isolated calcareous bodies, and on carefully 

 examining the fine processes of the canals, 

 no doubt could exist that they were only 

 interspaces or gaps. On carefully adjusting 

 the focus, in fact, it was obvious that one of 

 these large calcareous globules is itself only 

 an agglomeration of many smaller globules, 

 and it could be observed that the gaps left 

 between the latter became the fine processes 

 of the tubules. From these facts, I believe 

 that the correct mode of conceiving the 

 growth of the substance of the spine is, to 

 suppose that the calcareous matter is excreted 

 from the vessels of the pulp, and then in all 

 probability combined with organic matter, 



runs into smaller masses ; these unite together 

 into larger ones, and become applied to the 

 inner surface of the central cavity, coalescing, 

 and thus adding to the thickness of the spine. 

 Between the calcareous globules, however, 

 canalicular gaps or tubules remain, which 

 form a connected network and communicate 

 with those branched cavities which already 

 exist in the spine. 



The scales of the Sharks and the dermal 

 spines of the Rays, then, (and I would draw 

 particular attention to this result,) are per- 

 fectly identical in structure with the teeth, 

 even to the absence of nerves in the pulp, 

 and must be united in the same structural 

 group. I have already (On the Skin of Fresh- 

 water Fishe-s, Zeitschrift fiir Wiss. Zool. B. 

 iii. H. 4.) pointed out the close affinity be- 

 tween the scales of a number of osseous 

 fishes and their teeth : and scales likewise 

 present globules of calcareous matter, which 

 become fused together to form the homo- 

 geneous substance of the scale. A process, 

 corresponding with that which occurs at the 

 surface of the pulp in the teeth and cutaneous 

 spines, here takes place from the surface of 

 the sac of the scale (Schuppentasche). The 

 scales of osseous fishes, the spines of the 

 Rays, and the scales of the Sharks, therefore, 

 all belong to the series of dental structures, 

 which in no respect interferes with the en- 

 trance of true bony tissue (like the " cement " 

 in the higher animals) into their composition, 

 as we find to be the case in the scales of the 

 Ganoids (Miiller), and in the truly bony semi- 

 canals which are attached to the scales of the 

 lateral lines of many fishes." * 



For the del ails of the various modes in which 

 Ganoin, true osseous tissue, and those va- 

 rieties of tubular, more or less dentine-like 

 tissues, to which Prof. Williamson has given 

 the names of " Lepidine and Kosmine," are 

 combined together in the scales of Ganoid 

 and Placoid fish, I must refer to that gentle- 

 man's memoirs, already so often cited. 



In the Ctenoid and Cycloid fishes there is 

 a superficial " Ganoin " layer, composed of 

 numerous thin structureless calcified laminae, 

 which are frequently thrown into folds, papillae 

 or spines. The deeper substance of the scale 

 is composed of a series of layers of a mem- 

 branous substance, each layer being composed 

 of parallel fibres which take a different direc- 

 tion from those of the superficial and subse- 

 quent layers, so that the fibres of alternate 

 layers cross diagonally. No endoplasts or 

 cells are ever distinguishable among the fibres. 

 In the deepest part of the scale these layers 

 are entirely membranous ; but in passing to- 

 wards the surface, minute lenticular masses 

 of calcareous matter make their appearance 

 in the membranous substance. As Prof. 

 Williamson justly states, these lenticular bodies 

 are not developed between the membranous 

 fibres and lamellae, but in them : " they com- 

 mence as a small calcareous atom, and in-. 



* Leydig : Rochen und Haie, 1852. 



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