THE THORAX. 367 



the true. The elasticity diminishes as years go on, until, at an 

 advanced age, they are calcified. Rib-cartilage is not infrequently 

 fibrous in character, and the cells are, as a rule, larger and collected 

 into groups of greater size than those of articular cartilage (Fig. 

 209). 



Respiratory Muscles. The muscles which are concerned 

 in the respiratory movements of the thorax may be divided into 

 three groups: (1) Of ordinary inspiration; (2) of forced inspira- 



Matrix. ir^t: 







' 



il 



FIG. 209. Hyaline cartilage (costal cartilage of the ox). Alcohol preparation ; 

 X 300. The cells are seen inclosed in their capsules. In the figure a are repre- 

 sented frequent but by no means characteristic radiate structures (Bohm and 

 Davidoff). 



tion; (3) of forced expiration. There are no muscles concerned 

 in ordinary expiration. 



Muscles of Ordinary Inspiration. These are diaphragm, scaleni, 

 external intercostals, internal intercostals (anterior portion), leva- 

 tores costarum. 



Th.e Diaphragm. This forms the lower boundary of the 

 thoracic cavity, separating it from that of the abdomen. It arises 

 from the whole interior surface of the thorax, and the fibers 

 which compose its muscular portion are inserted into the central 

 or cordiform tendon. Through the diaphragm pass the vena cava, 

 the esophagus, and the aorta. 



Nerve-supply. The phrenic nerves and the phrenic plexus 

 of the sympathetic. 



Scaleni. The scalenus anticus is a muscle which arises from 

 the anterior tubercles of the transverse processes of the third, 

 fourth, fifth, and sixth cervical vertebrae, and is inserted into the 



