COMPARATIVE ANATOMY. 



cd 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 ma'rrmv, are filled with air. This 

 organization unites the advantages of 

 strength and lightness. 



Crustaceous animals, (crab, lobster, 

 Stc.) 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 drtacherl, 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 the increased deposition 

 of calcareous matter. 



Some of the mollusca 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, anu slightly convex on its two 

 surfaces, commonly known by the name 

 the cuttlefish-bone, contained in the 

 substance of its body. I\ 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. 



SKELETON OF MAMMALIA. 



The form of the different mammalia, 

 particularly the four footed ones, varies 

 considerably; and their skeletons must 

 be marked by corresponding differen- 

 ces. Yet these varieties may be includ- 



ed, at least for the greatest part, under 

 the following peculiarities ; which serve 

 to distinguish their skeletons from those 

 of birds. 



The skeletons of 

 mammalia possess : 



1. A sku-11 with 

 genuine sutures, at 

 least with very few 

 exceptions ; as per- 

 haps the elephant, 

 and the duck-billed 

 animal, (ornitho- 

 rhyncus.) 



2. Jaws furnish- 

 ed with teeth. 



Except the ant- 

 eaters, the mam's, 

 the balaena (whale.) 



3. An upper jaw, 

 which does not 

 move. 



Those of birds are 

 distinguished by : 



1. A skull which 

 has not real sutures, 

 at least in the adult. 



2. A bill without 

 teeth. 



3. An upper jaw, 

 which does move. 



There are some 

 exceptions, viz. the 

 rhinoceros bird. 



4. An os inter- 4. No os inter- 

 maxillare. maxillare. 



5. Two occipital 5. A single occi- 

 condyles. pital condyle. 



6. Seven cervical 6. More than se- 

 vertebrae. ven cervical verte- 



Except the three- brae, 

 toed sloth, and some 

 cetacea. 



7. Moveable dor- 

 sal vertebrae. 



8. A pelvis closed 

 in front. 



Except the ant- 

 eaters ; which have 

 it open : and the 

 cetacea, which have 

 none. 



7. Motionless dor- 

 sal vertebrae. 



8. A pelvis open 

 anteriorly. 



Except the os- 

 trich. 



9. True clavicles 9. Clavicles con- 

 in a few genera stantly ; and almost 

 only. as universally the 



forklike bone. 



The structure of the cranium presents 

 a very remarkable singularity in the 

 elephant. Its two tables are separated 

 from each other, to a considerable extent, 

 by numerous bony processes; between 



