COMPARATIVE OSTEOLOGY.
It has been asserted, that the bones in some instances have not their ordinary white colour. Thus the amedabad finch, (fringilla amandava,) and the golden pheasant, have been said to possess yellow bones ; but this is not true. In the garpike (esox the bones are green ; and in some varieties of the com mon fowl in the East Indies they are black ; but this colour is said by Mr. Hun ter to reside in the periosteum.
The opinion of Aristotle, that the bones of the lion had no marrow, is totally un founded.
The bones of the cranium are much more completely ossified at the time of birth, in the mammalia, than in man. In the former the fontanells are hardly dis cernible. When we compare the pelvis, and the whole mechanism of parturition in the woman, with those of the female quadruped, the cause of this difference appears; we then discover, why the yield ing and over lapping of the large bone of the cranium, which is chiefly effected by the fontanells, is only requir ed to facilitate the birth of the human foetus.
The skeleton remains constantly carti laginous in some animals ; such as the skate, shark, sturgeon, and all those fish es, which, from this circumstance, have been denominated cartilaginous. The bones of birds are almost universally hol low ; but their cavities, which never contain marrow, are filled with air. This organization unites the advantages of strength and lightness.
Crustaceous animals, (crab, lobster, &c.) have a skeleton which surrounds and contains their soft parts, and .which serves at the same time the purposes of a skin When it has attained its per fect consistence, it grows no more : but as the soft parts still increase, the shell separates, and is detached, being suc ceeded by a larger one. This new co
vering is partly formed before the other separates : it is at first soft, sensible, and vascular ; but it speedily acquires a hard consistence by Ihe increased deposition of calcareous matter.
Some of the molluscs have hard parts in the interior of their body. The com mon cuttlefish (sepia officinalis) has a white, firm, and calcareous mass, of an oval form, and slightly convex on its two surfaces, commonly known by the name the cuttlefish-bone, contained in the substance of its body. It has no connec tion with any soft, part, whence it ap pears completely as a foreign body : no vessel nor nerve can be`perceived to en ter it ; nor does it receive the attach ment of any tendon. In the calmar (se pia loligo) this body resembles horn in its appearance ; it is transparent, hard, and brittle. Its form resembles that of a leaf, except that it is larger ; and some times that of a sword blade. These parts must grow like shells, by the simple ad dition of successive layers.
In the vertebral animals, the bony parts of the body are composed of a ge latinous substance, united to phosphate of lime. But in the lower orders of ani mals, the hard parts are composed chiefly or entirely of carbonate of lime. This is the case with the shells of all the testacea.