456 



THE BLOOD- VASCULAR SYSTEM. 



Capillaries 



'ulmonary 

 Veins 



Speaking generally, the arteries may be said to contain pure, and the veins 

 impure, blood. This is true of the systemic, but not of the pulmonary, vessels, 

 since it has been seen that the impure blood is conveyed from the heart to the 



lungs by the pulmonary arteries, and the 

 pure blood returned from the lungs to the 

 heart by the pulmonary veins. Arteries, 

 therefore, must be defined as vessels which 

 convey blood from the heart, and veins as 

 vessels which return blood to the heart. 



The heart and lungs are contained 

 within the cavity of the thorax, the Avails 

 of which afford them protection. The 

 heart lies between the two ,lungs, and is 

 there enclosed within a membranous bag, 

 the pericardium, Avhile each lung is in- 

 vested by a serous membrane, the pleura. 

 The skeleton of the thorax and the shape 

 and boundaries of the cavity will be de- 

 scribed in the section on General Anatomy. 

 The Cavity of the Thorax. The capa- 

 city of the cavity of the thorax does not 

 correspond with its apparent size exter- 

 nally, because (1) the space enclosed by 

 the lower ribs is occupied by some of the 

 abdominal viscera; and (2) the cavity ex- 

 tends above the first rib into the neck. 

 The size of the cavity of the thorax is 

 constantly varying during life with the 

 movements of the ribs and Diaphragm, 

 and with the degree of distention of the 

 abdominal viscera. From the collapsed 

 state of the lungs, as seen when the thorax 

 is opened, in the dead body, it would 

 appear as if the viscera only partly filled 

 the cavity of the thorax, but during life 

 there is no vacant space, that which is 

 seen after death being filled up by the 

 expanded lungs. 



The Upper Opening of the Thorax. The 

 parts which pass through the upper opening 

 of the thorax are, from before backward in or near the middle line, the Sterno- 

 hyoid and Sterno-thyroid muscles, the remains of the thymus gland, the trachea, 

 oesophagus, thoracic duct, the inferior thyroid veins, and the Longus colli muscle 

 of each side ; at the sides, the innominate artery, the left common carotid and left 

 subclavian arteries, the internal mammary and superior intercostal arteries, the 

 right and left innominate veins, the pneumogastric, cardiac, phrenic, and sympa- 

 thetic nerves, the anterior branch of the first dorsal nerve, and the recurrent 

 laryngeal nerve of the left side. The apex of each lung, covered by the pleura, 

 also projects through this aperture, a little above the margin of the first rib. 



The Lower Opening of the Thorax is wider transversely than from before back- 

 ward. It slopes obliquely downward and backward, so that the cavity of the 

 thorax is much deeper behind than in front. The Diaphragm (see page 325) closes 

 in the opening, forming the floor of the thorax. The floor is flatter at the centre 

 than at the sides, and is higher on the right side than on the left, corresponding in 

 the dead body to the upper border of the fifth costal cartilage on the former, and 

 to the corresponding part of the sixth costal cartilage on the latter. From the 

 highest point on each side the floor slopes suddenly downward to the attachment 



FIG. 272. Diagram to show the course of the 

 circulation of the blood. 



