174 MANUAL OF ANIMAL BIOLOGY 



as a type of connective tissue in which the cells have become some- 

 what enlarged and atypical in shape as a result of fat storage. 



Cartilage is a highly developed connective tissue, which is 

 always abundant in the Vertebrate embryo. In the lowest classes, 

 it remains as the permanent skeleton, but in the higher types it is 

 later replaced by true bone. Some of the cartilaginous material, 

 however, remains unchanged throughout life. Cartilage is marked 

 by the presence of an exceptionally large amount of transparent, 

 intercellular ground substance which possesses great strength. 

 The cartilage cells are embedded in this matrix in numerous cavi- 

 ties, known as lacunae, each of which may contain one, two, or 

 even more cells. There are various kinds of cartilage, depending 

 upon the character of the ground substance. In hyaline carti- 

 lage the ground substance is homogeneous, but in the mixed and 

 fibrous types of cartilage the ground substance shows a marked 

 fibrillar nature due to the presence of either white or elastic fibers 

 or both. Hyaline cartilage is the more common type, and it is 

 found in the joints, in the breast bone, between the vertebrae, and 

 in a number of other places in the body. ( W. fs. 32, C; 100.) 



Bone. The greater portion of the bone tissue of the body, as 

 has been noted, is first formed as cartilage. Such bones are, there- 

 fore, known as cartilage bones in distinction to a less frequent 

 type, known as membrane bones, in which the cartilage stage is 

 lacking, and the bone tissue develops by an ossification of soft 

 membranes. The general structure of mature bone shows a re- 

 semblance to cartilage in that the bone cells, or osteoblasts, 

 are embedded in cavities in the matrix in somewhat the same way 

 as in cartilage. The microscopic structure of bone can be seen to 

 good advantage in a properly prepared transverse section through 

 one of the leg bones, for example, the femur. Such a section shows 

 that the bone is not a solid rod throughout, but consists of an 

 outer cylinder of bone tissue enclosing an internal cavity which 

 runs the length of the bone. In life, the cavity is filled with a soft, 

 highly vascularized tissue, the bone marrow. The bone tissue is 

 covered on the outside by a living connective tissue sheath, the 

 periosteum, and is "arranged in concentric layers, or lamellae, 

 which contain numerous lacunae in which lie the bone cells. 

 From the lacunae, fine branching tubes, or canaliculi, containing 

 processes from the bone cells, are given off which extend in all 

 directions and anastomose with the canaliculi of neighboring 



