LIGAMENTS. 39 



Some bones are found with their animal ingredients remaining 

 others are fossilised. The phosphate of lime loses its phosphoric 

 acid, and the earth remains incorruptible, while the soft animal 

 matter decomposes and dissipates. The bone in this condition 

 may become fossilised ; silicious earth, or lime combined with iron, 

 may pass, by infiltration, into the interstices of the original earthy 

 matter, and in this state it is permanent as the solid rock. 



There is preserved in the Royal Museum of Madrid, a skeleton 

 of the enormous megatherium of Cuvier. It is supposed that the 

 animal was seven feet in height; for its femur is three times the 

 diameter, and its pelvis twice the breadth of that of an elephant. 



CHAPTER IV. 



LIGAMENTS. 



In the lowest grades of animal beings, cartilages, synovial mem- 

 branes, capsular or accessary ligaments, scarcely exist at the joints. 

 We here rind the movable points formed of a tough connecting 

 material, which, by its elasticity, admits of the limited motions 

 required. 



No ligamentary apparatus appears in the soft, gelatinous animal- 

 cules ; but the silicious and calcareous spicula of the poripherous 

 radiata are supported by a tough, elastic species of cellular tissue. 

 The plates composing the shells of the echinida are united by 

 sutures, and the enarthroidal joints of the spine by capsules, and 

 often by a ligamentum teres, as in the cidaris. In the larger Crus- 

 tacea and coleopterous insects portions of the skeleton are locked 

 into each other, where they move securely, but to a limited extent, 

 in the angular direction without ligaments. The shells of the con- 

 chifera are united by their locking teeth, and by a strong, tough 

 ligament, which, by its elasticity, constantly tends to the separation 

 of the bivalves. 



In the soft, flexible skeletons of the cartilaginous fishes, the liga- 

 ments are few, and confined to the organs for mastication and pro- 

 gressive motion. But in the osseous fishes the ligaments of the 

 spine are white, fibrous, dense, and highly elastic ; and here, for the 

 first time, we meet the contiguous ends of bones incrusted with car- 

 tilage. In the amphibia, and in the reptiles, the bodies of the vertebrae 

 are united by enarthroses, furnished with strong fibrous capsules and 

 synovial secretion. Here we have external fibrous bands, inter- 

 spinous ligaments, and occasionally loose cartilages in the joints. 

 The capsular ligaments of birds are thin and strong : their cartilages 

 of incrustation are also thin, but their joints are freely supplied 

 with synovia and their hip joint furnished with a strong ligamen- 

 tum teres. 



