198 Mammalia, 



to the inner surface of tlie sternum : and if the student will 

 hold up gently with the forceps this upper portion, he will 

 see the sort of inverted A thus formed ; the stem being 

 mediastinum anterior ; the branches, the internal side of the 

 pleural sac on each side ; the space between the branches being 

 occupied by the pericardium, or membrane surrounding the 

 heart. On gently pulling down the diaphragm, the thin 

 white cords of the Phrenic nerves will be apparent ; coming 

 down nearly in a straight line ventrad of the roots of the 

 Lungs and by the side of the pericardium {i.e., between it and 

 the mediastinal portion of the pleura), to the Diaphragm. 



Exposure of Yessels, &c., in the Neck, When the 

 Student made the incision in the median line through 

 the skin from the symphysis of the lower jaw, and 

 reflected the skin on either side ; he by this means exposed 

 the sub- maxillary gland, as a large pinkish mass. This should 

 be divided in the middle line, as also the sterno-mastoid, 

 sterno-thyroid, and other muscles more or less closely connected 

 with it in the neck, and reflected to each side, by which means 

 the trachea will be exposed, and the vessels and nerves on either 

 side of the neck. 



And here the Student will observe first, that it is the 

 external, and not the internal jugular that is the main 

 trunk by which the blood from the interior of the skull 

 returns to the heart in these Rodents, and which by its con- 

 fluence with the vein from the anterior limb constitutes the 

 vena cava descendens : the internal jugular being here repre- 

 sented only by a small vein passing dorsad of the clavicle, and 

 forming the third confluent tributary in the same relative 

 position as in Man ; and secondly, that in forming this union 

 the external jugular crosses the clavicle on the outside, and then 

 together with the subclavian penetrates into the thoracic 

 cavity in the interval between the clavicle and the first rib, 

 the two veins uniting as they enter the cavity. Thus in two 



