Practical Course. 109 
(4) If the specimen isa female notice the thick-walled organs 
(uteri) which bifurcate behind the bladder and vary very 
much at different times of the year. If the specimen is 
a male, notice the paired testes, which can sometimes be 
pulled back into the abdominal cavity from the scrotal sacs. 
Notice also the bilobed uterus masculinus behind the 
bladder, which receives the vasa deferentia from the 
testes and itself communicates with the urogenital canal. 
Cut through the ribs on either side carefully with scissors and dissect 
off the breast-bone (sternum), together with the whole of the frout wall 
of the thoracic cavity, so as to expose the organs which lie below. 
Observe that the thoracic cavity is completely shut off from the abdo- 
minal cavity, and is lined with glistening membrane. In the middle lies 
the large rounded heart enclosed in transparent membrane (pericardium), 
while on either side are the spongy lungs, 
Clear away the muscles in front, soas to make out the wind-pipe 
(trachea), through which air passes into the lungs. It lies in the middle 
line of the neck and can be immediately recognised by the thickened 
cartilaginous rings by which it is supported, 
Clear away the glandular matter (thymus) from in front of the thora- 
cie cavity, dissect off the membrane in which the heart is enveloped, and 
clean the vessels at the base of the heart. By pressing the heart and the 
lungs apart a white nerve chord (phrenic nerve) can be seen on either side. 
‘These nerves are branches of the fourth cervical nerves and supply the 
muscular partition (diaphragm) which separates the thoracic and abdo- 
minal cavities. 
Examine the heart aud make out— 
(1) The thick muscular ventricles which occupy all the posterior 
portion of the organ, the partition between the right 
and left halves not being very plainly visible externally. 
(2) The right and left auricles, which are two thin-walled chambers 
lying in front of the right and left ventricles. To see the 
auricles plainly the heart should be turned over a little 
first to one side and then to the other. The right auricle 
receives the blood from all parts of the body except the 
lungs, and passes it on into the right ventricle, The right 
ventricle drives the blood to the lungs, whence it returns to 
the left auricle, which passes it on into the left ventricle. 
‘lhe left ventricle drives the blood to all parts of the body, 
whence it finds its way back to the right auricle, thus com- 
pleting the circle, 
The chief vessels in connection with the heart are— 
(1) The right and left vessels (anterior vena cava) by which the 
blood returns to the right auricle from the anterior portion 
