TEE THORAX. 



97 



external surface of this bony cas^e. For this purpose it is divided into six 

 regions — a superwr plane, an inferior phme, two lateral planes, a base, and a 

 summit. 



Planes. — The snperior plane is separated into two portions by the spinous 

 processes of the dorsal vertebrse ; each forms, with these spinous processes, the 

 costo-vertebral furrow, intended to lodge the majority of the muscles belonging 

 to the spinal region of the back and loins. The inferior plane, less extensive 

 than the preceding, offers : 1. On the median line, the cariniform and xiphoid 

 cartilages of the sternum ; 2. On the sides, the chondro-sternal articulations, 

 and the cartilages of prolongment of the true ribs. The lateral planes are con- 

 vex and wider at their middle part than in front or behind, and exhibit the 

 intercostal spaces. They serve to give support, anteriorly, to the superior 

 segments of the two anterior limbs. 



Base. — This is circumscribed by the posterior border of the last rib, and by 

 the cartilages of all the asternal ribs ; it is cut obliquely downwards and for- 

 wards, and gives attachment, by its internal 



circumference, to the diaphragm — a muscle tig. 58. 



which separates the thoracic from the ab- 

 dominal cavity. 



Summit. — It occupies the anterior por- 

 tion of the thorax, and presents an oval 

 opening, elongated vertically, situated be- 

 tween the two first ribs. This opening 

 constitutes the entrance to the chest, and 

 gives admission to the trachea, the oeso- 

 phagus, and important vessels and nerves. 



Comparison of the Thorax of Man with 

 THAT op the Domesticated Animals. 



1. Sternum. 



The sternum of Man is flatteneil before and 

 behind, and diminishes in width from above to be- 

 low. The xiphoid appendage is narrow, and single 

 or bifid. Be8i<ie8 the articular surfaces for the ribs, 

 there are found on tlie ujiper end two lateral 

 notches for articulation with the clavicles. 



2. Ribs. 



Of the twelve ribs in Man, seven are sternals 

 and fivf asternals. They are short, narrow, and 

 mucli incurvated, especially the first ones. In each 

 rib the curvature is mori' niarkid in the posterior 

 fourth or fifth, than in tin; anterior three-fourths 

 or four-fifths ; tiiis sudden change of curvature is 

 indicated in the external face by a kind of inflection 

 and thickening, called tiie angle of the ribs. The 

 prolonging cartilages of the eleventh and twelfth 

 ribs are short, and are lost in the substance of the 

 are termed ihe floating {or falst) ribs (Fig. 58). 



thorax of man (anterior face). 



1, Superior piece of the sternum ; 2, 

 middle piece, or body ; 3, inferior piece, 

 or ensiform cartilage; 4, first dorsal 

 vertehra ; 5, last dorsal vertebra ; 6, 

 first rib; 7, its head; 8, its neck, rest- 

 ing against the transverse process of the 

 first dorsal vertebra ; 9, its tubercle ; 

 10, seventh, or last true rib ; 11, costal 

 cartilages of the true ribs ; 12, the last 

 two false or floating ribs ; 13, the groove 

 along the lower border of the rib. 



abdominal parietes ; for this reason they 



Aeticle IV. — Anterior Limbs. 



The anterior (pectoral or thoracic) limb is divided into four secondary regions — 

 the shoulder, arm, forearm, and fore foot or hand. 



