THE BONES Of THE TRUNK 55 



aperture transmits the following structures : the apical parts o! 

 the lungs and pleura;, the trachea and oesophagus, the pneumo- 

 gastric, sympathetic, and phr^iic nerves, the terminal part of the 

 innominate artery, the left common carotid and left subclavian 

 arteries, and the right and left innominate veins. In early life it 

 also transmits the thymus gland. 



The inferior aperture is of large size, and is boimded posteriorly by 

 the twelfth thoracic body, laterally by the twelfth rib at either side, 

 and anteriorly by a line, at either side, connecting the costal cartilages 

 from the twelfth to the seventh inclusive. These two lines constitute 

 the subcostal angle, within which the metastemum is situated. The 

 inferior aperture is occupied by the diaphragm, which presents 

 certain openings for the passage of important structures. 



The cavity, on either side of the thoracic bodies, presents an 

 elongated groove, called the pulmonary groove, which lodges the 

 thick posterior border of a lung. The cavity has the following 

 diameters, namely, vertical, transverse, and antero-posterior. The 

 vertical diameter extends from the superior aperture to the inferior. 

 The transverse diameter extends from the centre of a given inter- 

 costal space to the centre of the corresponding space of the opposite 

 side. The antero-posterior diameter extends from the anterior to 

 the posterior wall, and is necessarily of less extent in the median line 

 than at either side, on accovmt of the projection formed by the 

 thoracic bodies, its increase at each side being due to the presence 

 of the pulmonary groove. The cavity is increased in all these 

 diameters during inspiration, and diminished during expiration. 



The intercostal spaces are eleven in number at either side. They 

 increase in length from the first to the fifth, and are occupied for 

 the greater part of their extent by the external and internal inter- 

 costal muscles. 



The thorax of the female is rather shorter than that of the male, 

 and is not so much flattened from before backwards. 



In early life the thorax is flattened from side to side, and its 

 height is relatively less than in the adult. 



The Notoehord and the Development of the Vertebral Column, 

 Ribs, and Sternum. 



Notoehord. 



The notoehord forms the primitive basis of the axial skeleton, and around 

 it the bodies of the vertebrae are developed. It is a soUd cylindrical rod of 

 cells, derived from the cephalic end of the primitive streak, and it occupies 

 the median line, corresponding to the centres of the bodies of the future 

 vertebrae. It Ues along the ventral aspect of the neural tube, which con- 

 stitutes the primitive tubular nervous sj-stem, and along the dorsal aspect 

 of the archenteron, or primitive intestinal cax-ity-. The cephalic end of the 

 notoehord is situated on the ventral aspect of the mid-brain, and corresponds 

 to the posterior part of the pituitary region of the base of the future cranium. 

 From this region it extends to the caudal end of the future axial skeleton, 



