112 ANATOMY OF UPPER EXTREMITY, ETC. 



opposite side, and, in front, with the tendinous origins of 

 the pectoral muscles. 



The costal cartilages are held in connection with the 

 anterior extremities of the ribs only by the periosteum. 



The cartilages of the sixth, seventh, and eighth ribs 

 articulate by their lower borders with the upper borders of 

 the cartilages next below. The articulation is lined with a 

 synovial membrane, and the connection is maintained 

 ligamentous fibres. The cartilage of the seventh, and 

 times also of the sixth rib, is attached to the ensiform car- 

 tilage by a band of variable size, called the costo-xiphou 

 ligament. 



ANTERIOR MEDIASTINUM. 



The space left between the two pulmonary cavities is 

 called the MEDIASTINUM; it extends from the summit of 

 the chest to the diaphragm, and from the sternum to the 

 spine. Although there is but one mediastinum, the terms 

 anterior and posterior have been applied respectively to 

 the portions in front of and behind the pericardium. 



The ANTERIOR MEDIASTINUM contains a quantity of loose 

 cellular tissue, the remains of the thymus gland* the peri- 

 cardium and heart, arch of the aorta, superior vena cava, 

 with the right and left innominate veins, bifurcation of the 

 trachea, pulmonary veins and arteries, and phrenic nerves. 



The THYMUS GLAND, in the adult subject, consists only 

 of a small quantity of cellulo-adipose tissue; sometimes no 

 trace of it is to be found. In children under two years of 

 age it is an organ of considerable size, reaching from just 

 below the thyroid bod} T , half-way to the diaphragm. 



The PHRENIC NERVES, the upper parts of which will have 

 been observed in the dissection of the neck (p. 56), where 

 they arise by filaments from the third, fourth, and fifth cer- 

 vical nerves, and descend upon the scalenus anticus muscle 

 to enter the chest, will here be seen passing through the 

 anterior mediastinum upon the sides of the pericardium, 

 between it and the pleura. Their course can be traced with- 

 out dissection, descending to the diaphragm, upon which 

 they ramify beneath the pleura, the two nerves anasto- 

 mosing on the under surface of the muscle by filaments, 

 which pass through its fibres. 



The VENJE INNOMINATE are found in the upper part of 

 the mediastinum. They are formed on each side by the 

 subclavian and internal jugular veins; the union of the 



