846 



OSSEOUS SYSTEM. (Coup. ANAT.) 



fact which is remarkable for its uniformity in 

 other parts of the skeleton and in other animals. 

 A separate centre of ossification is developed, a 

 new source of nutrition is conveyed directly to 

 the extremity of the part a new bone is formed 

 from the end of the spinous process." But 

 even this dilatable explanation is by no means 

 sufficient for the required purpose, for having 

 developed in this way the interspinous bones 

 from the spinal elements of the vertebrae, the 

 fin rays themselves are referred to the same 

 source, and thus materials are afforded for com- 

 plicating the endoskeleton ad libitum, upon the 

 simple supposition that when any element be- 

 comes inordinately developed, it can develope 

 other elements to eke it out. 



Geoffrey adopted another mode of explaining 

 the origin of the supernumerary bones that 

 support the dorsal and anal fins of fishes. Sup- 

 posing the upper and lower vertebral spines 

 (i. e. the neuro-spines and haerno spines) to be 

 each composed of two elements conjoined in 

 the mesial line, he asserts that, instead of re- 

 maining side by side, one half of the spine is 

 removed and placed above the other to form 

 the interspinous bone. Yet even this would 

 by no means get over half the difficulty, for 

 the fin rays themselves remain to be accounted 

 for, and where are elements to be procured for 

 the construction of these ? Moreover, as Cu- 

 vier remarks, it by no means unfrequently 

 happens that several interspinous bones belong 

 to a single vertebral spine, a circumstance 

 which is quite incompatible with the supposi- 

 tion that any dismemberment of the spine can 

 account for their presence. 



Failing, therefore, to find any materials for 

 the construction of the bones we have enume- 

 rated among any known elements of the endo- 

 skeleton, we are compelled to look elsewhere, 

 and shall soon find, by tracing the exoskeleton 

 of Fishes through the different aspects under 

 which it offers itself, abundant means of supply- 

 ing all deficiencies. 



The scales that usually invest the bodies of 

 ordinary Fishes would certainly appear at first 

 sight to have no relationship whatever with the 

 osseous system, as neither in texture nor mode 

 of growth do they at all resemble bone, being 

 simply composed of layers of epidermis se- 

 creted one after the other until they attain the 

 required thickness. But these epidermic scales, 

 if but very slightly exaggerated, become sus- 

 ceptible of such varieties of form and structure 

 that they often entirely lose their nature, and 

 becoming solidified until they emulate true 

 bone in hardness and compactness of structure, 

 are often converted into weapons of defence 

 or attack of very diversified descriptions; the 

 dense and bone-like armour of the Ostracious, 

 the formidable spines of the Diodon, and even 

 the crystalline tooth-like points that stud the 

 skins of Sharks and Rays, forming what is called 

 shaureen, being mere modifications of the same 

 cuticular appendages. Having advanced thus 

 far in tracing the changeable character of the 

 scale of a fish, adapting it to various functions, 

 we are quite prepared to admit other important 

 facts of still greater interest. It is only neces- 



sary to examine the spines met with upon the 

 back of a common Skate or Thornback to per- 

 ceive that they are of very different character in 

 different parts of the surface of the body. The 

 minute scale-like points are at times converted 

 into large hooks fixed upon the surface of the 

 skin, which really become formidable defences. 

 On approaching the mouth they become again 

 so much reduced in size as to represent an ex- 

 ceedingly fine tessellated pavement, which 

 covers the lips and passes even into the interior 

 of the mouth. On reaching the margins of the 

 upper and lower jaw their appearance again 

 becomes changed ; they are increased in size 

 and hardness, being in fact converted into 

 teeth which pave the whole surface of the jaw, 

 covering it with osseous plates, or powerful 

 hooks, or cutting teeth, such as the Shark pos- 

 sesses; but these teeth are still quite uncon- 

 nected with the jaw and may be easily stripped 

 off with the cuticle, of which, indeed, they form 

 a part. Even the tongue itself is covered with 

 similar plates of hard substance, smaller in size 

 indeed, but in every thing comparable to the 

 teeth both in character and mode of growth. 

 Finding that the teeth in their simplest form 

 are merely epidermic structures, nothing would 

 be more easy than to point out a long series of 

 almost insensible gradations through which 

 they become more and more decidedly con- 

 nected with the bones of the endoskeleton, 

 until at length they absolutely become implanted 

 into it, and fixed to the jaw-bones as intimately 

 as if they were really themselves portions of the 

 true osseous skeleton. The ligamentous bonds 

 of union between the teeth and jaw-bones of 

 Lopfuus, the soldering together of the teeth, 

 and the numerous bones which, in Fishes, are 

 made to support dental appendages, and the 

 gradual appearance of alveolar depressions in 

 the jaws of the higher lleptilia, are all so many 

 steps of progressive approximation, the interme- 

 diate phases of which the scientific reader will 

 easily supply. These facts therefore satisfactorily 

 prove that, as far as the teeth are concerned at 

 least, the exoskeleton and endoskeleton are so 

 nearly approximated in texture that they ac- 

 tually become appendages one of the other, as 

 the teeth are infixed into the jaw-bones that 

 support them. 



Having thus convinced ourselves that in the 

 case of the teeth the cuticular and osseous 

 systems become articulated together, or so con- 

 solidated as only to be distinguishable from 

 the microscopic texture that they respectively 

 present, we are prepared with greater confidence 

 to expect similar phenomena in other parts of 

 the body, and to find the exoskeleton and endo- 

 skeleton to a certain extent vicarious in function 

 and interchangeable with each other. 



No one will deny that the spines of the 

 common Sticklebacks (Gasterosteus), or the 

 bony-looking weapons affixed to the root of the 

 tail of the Sting-ruy, arc cuticular in their na- 

 ture and mere derivations from the exoskeleton ; 

 yet we have only to advance one step further, 

 and we find spines in every way similar in 

 their nature absolutely articulated by curious 

 and most beautiful moveable joints to different 



