180 ANTHROPOLOGY. © 
being performed by the small bronchial arteries which proceed directly: 
from the aorta, and from independent plexuses in the lungs which yet com- 
municate with those of the pulmonary artery. The returning bronchial 
veins, too, lead partly to the superior vena cava and partly to the pulmonary: 
veins, so that there is always a small quantity of black blood mixed with: 
the arterial in the left auricle. The nerves destined to the lungs pro- 
ceed from the vagus and sympathetic. The lungs have comparatively 
little sensibility, as even in extensive pulmonary diseases but little aac is 
felt. 
Pl. 180, fig. 4, larynx, trachea, pericardium, and lungs, from before: 
’, larynx; °*, trachea; **, lungs; °, pericardium; °, superior vena cava; 
", arteria innominata; *, left carotid artery; °, left subclavian artery. — 
The organs contained within the thoracic cavity are inclosed by three 
completely closed serous sacs, the two pleurz and the pericardium. The 
latter has already been described with the heart. Hach pleura is a short 
sac of a conical shape, and contains only the serous vapor it exhales; 
although the lung appears within the cavity, it is really external to it or 
behind it. That portion of each which invests the lungs is called pleura 
pulmonalis, that connected with the parietes of the thorax being the pleura 
costalis or parvetalis. ‘The relations of the lungs to the pleura may be better 
understood by supposing the latter at first to occupy the thorax exclusively. 
They will then constitute two bags, in contact with each other by their 
inner and opposed faces. If now the lungs are considered to be developed . 
on the outside of the opposed surfaces of each bag, they will force this out 
towards the exterior, and finally connect each bag into two contiguous. 
lamine in contact with each other, the one the costal and the other the 
pulmonary pleura. The small space left anteriorly and posterior to the 
sternum is called the anterior mediastinum ; that posterior and in front of 
the vertebrze is the posterior mediastinum. The space between the two, 
the middle mediastinum, contains the lungs and heart. The anterior medias- 
tinum is wider superiorly and inferiorly than in the centre, and is somewhat 
X-shaped; the superior portion contains the origin of the sterno-hyoid and 
thyroid muscles, and the remains of the thymus gland. The posterior 
mediastinum is longer than the anterior, and includes the cesophagus, the 
larger blood-vessels, and the thoracic duct. 
Pl. 180, fig. 8, cross-section of the thorax to exhibit the course of the 
pleurz: ’, heart within its pericardium; * *, substance of the lungs; +, right 
pleura arising from the ribs and their cartilages; it bends back along the 
sternum, leaves the anterior mediastinum, *, between it and the left: pleura, 
passes over the pericardium, embraces the pulmonary vessels, passes over 
the lung, bends back again posteriorly, ’, the posterior mediastinum being 
formed between it and the left pleura as before. 
1. THE THYROID Bopy oR GLAND is a large, soft, red mass, of a crescentic 
shape, and lying on the trachea and sides of the larynx. It consists of two 
large pyramidal portions, called lateral lobes, connected by a narrow slip, 
the middle lobe or isthmus. The thyroid body is surrounded by a fine tissue ; 
it is of a soft spongy texture, the cells containing a yellow fluid. Four 
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