OF THE VISCERA. 475 



and infundibuliform in the upper stratum of the derma, but soon becomes 

 uniform and soft. The infundibuliform projection is drawn out from the 

 duct when the epiderma is removed, and may be perceived on the under 

 surface of the latter as a nipple-shaped cone. A good view of the sudo- 

 riferous ducts is obtained by gently separating the epiderma of a portion 

 of decomposing skin ; or they may be better seen by scalding a piece of 

 skin, and then withdrawing the epiderma from the derma. In both cases 

 it is the lining sheath of epiderma which is drawn out from the duct. 



CHAPTER XL 



OF THE VISCERA. 



THAT part of the science of anatomy which treats of the viscera is 

 named splanchnology, from the Greek words ovXa^vov, viscus, and Xo'^oj. 

 The viscera of the human body are situated in the three great internal 

 cavities : cranio-spinal, thorax, and abdomen. The viscera of the cranio- 

 spinal cavity, namely, the brain and spinal cord, with the principal organs 

 of sense, have been already described, in conjunction with the nervous 

 system. The viscera of the chest are : the central organ of circulation, 

 the heart ; the organs of respiration, the lungs ; and the thymus gland. 

 The abdominal viscera admit of a subdivision into those which properly 

 beiong to that cavity, viz., the alimentary canal, liver, pancreas, spleen, 

 kidneys, and supra- renal capsules ; and those of the pelvis ; the bladder 

 and internal organs of generation. 



THORAX. 



The thorax is the conical cavity, situated at the upper part of the trunk 

 of the body ; it is narrow above and broad below, and is bounded by the 

 sternum, six superior costal cartilages, ribs, and intercostal muscles in 

 front ; laterally , by the ribs and intercostal muscles ; and, behind, by the 

 same structures, and by the vertebral column, as low down as the upper 

 border of the last rib and the first lumbar vertebra ; superiorly ', by the 

 thoracic fascia and first rib ; and inferiorly, by the diaphragm. This 

 cavity is much deeper on the posterior than on the anterior wall, in con- 

 sequence of the obliquity of the diaphragm, and contains the heart en- 

 closed in its pericardium, with the great vessels ; the lungs, with their 

 serous coverings, the pleura ; the esophagus ; some important nerves; 

 and, in the foetus, the thymus gland. 



THE HEART. 



The central organ of circulation, the heart, is situated between the two 

 layers of pleura which constitute the mediastinum, and is enclosed in a 

 proper membrane, the pericardium. 



Pericardium. The pericardium is_L.fibLO-serous membrane like the 

 dura mater, and resembles that membrane in deriving its serous layer from 

 the reflected serous membrane of the viscus which it encloses. It consists, 

 therefore, of two layers, an external fibrous and an internal serous. The 



