Practical Course. 101 
breast-bone (sternum), to which the chief muscles of flight are attached, 
the situation of the transverse slit-like cloacal aperture below the tail, 
where the digestive, excretory and reproductive organs all terminate, and 
the aperture of the uropygial gland, which lies in the middle line above 
the tail. ‘ 
Dissect off theskin from one side of the breast, so as to expose the 
first or great pectoral muscle, which is the chief agent in depressing the 
wing in flight. It is attached at one end along the sternum and collar- 
bone (clavicle), whence its fibres converge to a tendon which is attached 
to the humerus. Cut through the muscle carefully about the middle of 
its length, taking care not to cut into the second pectoral muscle, which 
lies below. Turn back the two flaps so as to see bow the muscle is 
attached, and clean the tendon by which it is connected with the humerus. 
Now dissect away all the basal portion of the muscle, so as to expose the 
collar-bone (clavicle) on one side, Continue the dissection on the other 
side, so as to see how the clavicles are united together in the middle line 
to form what is popularly known as the merry-thought. 
On the side from which the great pectoral muscle has been com- 
pletely removed, you will now have exposed the second pectoral muscle, 
The second pectoral is very similar both in its method of attachment and 
in its shape to the great pectoral, but its tendon is very much longer. 
It passes through a pulley-like arrangement to its place of attachment at 
the back of the shoulder-joint. It thus serves to elevate the wing in 
flight, as can be readily seen on pulling the muscle with your forceps. 
You will now be able, without much difficulty, to recognise the chief 
bones of the pectoral girdle, which is a very characteristic feature in the 
anatomy of birds. It ecnsists of the following :— 
(1) The scapula, which is a long flattened bone lying behind the 
shoulder above the ribs. 
(2) The eoracord, which is a stout straight bone running from the 
shoulder to the front end of the sternum. 
(3) Theclavicle, which is ossified to the corresponding bone on the 
other side of the body, the two together making up the 
merry-thought, which we have already examined. 
You will also be able to make out the nature of the ventral aspect: of 
the sternum and ribs, ; 
Now dissect off the whole of the sternum, cutting through the ribs 
on either side and thé coracoids and clavicles in front with a pair of 
scissors, taking great care not to injure any of the organs which lie 
below. You will now have exposed the following organs :— 
(1) The ends of the posterior abdominal air-sacs. These are sacs 
of transparent membrane to be found on either side imme- 
diately under the posterior ribs ; they should be looked for 
