ENDOSKELETON AND VOLUNTARY MUSCLE 



73 



with proximal tarsals. Distal to the tibiotarsal is another 

 cylindrical bone, the tarsometatarsal, formed by fusion of distal 

 tarsals with metatarsals. As a result the ankle joint in birds is 

 intertarsal instead of between the tarsals and the tibia and 

 fibula. This condition also occurs in reptiles. Four toes are 

 present in the fowl, digit V being absent. 



Histology of Cartilage and Bone. — Bone and cartilage are 

 forms of connective tissue. Cartilage is composed of cells 

 embedded in a matrix of solid material that is a mixture of 

 collagen (a substance also found in white connective tissue 

 fibers), chondroitin- sulphuric acid in a combined form, and 

 albuminoid substances. Hyaline cartilage, which is translucent 



Fig. 45. — Cartilage, larval salamander. X 300. 



and pale gray in color, forms the articulating surfaces of bones, the 

 nasal cartilages, the costal (rib) cartilages, and the rings of the 

 trachea (windpipe) and bronchial tubes. Elastic cartilage is 

 yellowish in color, owing to the presence of yellow elastic fibers. 

 It occurs in the external ear of mammals. Fibro cartilage contains 

 in its matrix dense connective tissue composed mainly of white 

 fibers. In man it is found in the disks between the centra of the 

 vertebrae. The matrix of cartilage is produced by the cells 

 embedded in it (Fig. 45). 



Bone resembles cartilage in the topographical and functional 

 relations of cells and matrix. The matrix of bone in the early 

 stages of its histogenesis is soft and at that time is composed of 

 collagen, osseomucoid, and usually some connective tissue fibers. 

 The bone hardens as a result of deposition of inorganic salts, of 

 which the most abundant is calcium phosphate. Cartilage bone 

 is bone that passes through a cartilage stage in its development. 

 Membrane bone, on the other hand, is formed directly from con- 



