Dissection of Common Rat. 197 



Removal of Sternum. The student should now carefully- 

 clean the ribs and sternum from the junction of the clavicle 

 with the manubrium*, down to the ensiform cartilage, until 

 he can distinctly verify each rib. The sternal half of the 

 diaphragm should then be separated from its attachment to 

 the ribs with the scissors, and allowed to fall back ; the student 

 taking special care not to detach anything from its thoracic 

 aspect, lest in doing so he separate the Phrenic nerves which 

 reach the diaphragm right and left of its centre. The Ribs 

 should then be divided on either side, following the line of 

 junction between the true ribs and the costal cartilages ; and the 

 clavicle dislocated from the manubrium. Great care will be ne- 

 cessary to avoid puncturing the vessels anteriorly. The inter- 

 nal surface of the thoracic wall should be freed from the lining 

 membrane (pleura) with the handle of the scalpel as it 

 gradually admits of separation from the enclosed organs ; this 

 done, the sternal wall of the thoracic cavity should be 

 removed. 



The bag of the Pleura being now opened, the cavity with 

 its contents will be ready for examination. Anteriorly is the 

 Sternal attachment of the sterno-mastoid muscle, and the 

 Thymus gland. Next is the Heart in its pericardium, and on 

 either side the Lungs. The size of the thoracic cavity varies 

 during life, both transversely and in depth, concomitantly 

 with the situation of the ribs and diaphragm in breathing. 

 The Pleura are two serous membranes or closed sacs, which 

 are reflected around the Lungs in the cavity of the thorax. 

 These pleural sacs approximate, but do not blend, along the 

 middle line ; and this median portion thus constructed of two 

 membranes is termed the mediastinum^ . The part of the 

 mediastinum in front of the Heart extends from the pericardium 



* Manubrium^ a handle : the Sternum being looked on as a short broad dirk, 

 the ensiform cartilage being the point. 



t Mediastinum, medius the middle ; sto, I stand. 



