336 THE TISSUES. 



Distribution in the Lower Animals. 



In the invertebrata true osseous tissue is never found ; the exter- 

 nal calcareous skeleton taking its place. 



In the other vertebrata, osseous tissue is more extensively distri- 

 buted than in man. It exists, 1, in the skin (of the armadillo, tor- 

 toise, lizard, and fishes) ; 2, in muscles and tendons (the diaphragma- 

 tic bone of the camel, lama, and porcupine, and the ossified tendons 

 of birds) ; 3, in the eye (the sclerotic ring of birds, chelonians and 

 saurians, and the bony scales of the sclerotic of many fishes); 4, in 

 the external portion of the nose (the proboscis of the pig and mole 

 and the os pronasale of the sloth); 5, in the tongue (ps entoglossum 

 of fishes and birds) ; 6, in the air-passages (the laryngeal, tracheal, 

 and bronchial bones of many birds); 7, in the sexual organs (penis- 

 bone of some mammalia) ; 8, and as additions to the skeleton (ossa 

 sterno-costalia of birds and some mammals). 



SECTION II. 



STRUCTURE OF THE BONES. 



The bones are usually divided into the long, 1 the short, and the 

 flat ; and the largest bones, especially the long ones, have the follow- 

 ing elements entering into their structure : 



1. Osseous tissue. 



2. Bloodvessels. 



3. Nerves. 



4. Marrow (adipose tissue). 



Besides, the external surface of the bones is covered at every 

 point by some one of the following structures: 



1. Periosteum. 



2. Articular cartilages, and interarticular fibro-cartilages. 



3. Synovial membranes. 



4. Insertions (or origins), of tendons or ligaments. 



The periosteum* has already been described in general (page 279); 

 it being the fibrous membrane which invests the bone, and in which 

 the vessels ramify before entering the substance of the latter. A 

 more definite description of it will be given in the order above. 



1 The long bones consist of the shaft (diaphysis) and the two extremities (epi- 

 physes) ; the structure of the latter being like that of the short bones. 



2 From 7Tf, around, and OO-TIOV, a bone. 



