660 
DR. PETTIGREW ON THE CIRCULATION. 
cate with the interior of the bones, and with as many as 
fifteen air-sacs. These sacs open and close rhythmically, and 
pass on relays of air at regular intervals. If the humeri or 
arm-bones of a bird be perforated, its trachea or windpipe 
may be tied without producing suffocation, respiration going 
on by the openings in the bones. The air-sacs have vessels 
ramifying on their surface, so that a certain proportion of the 
blood is aerated in them, as in the lungs themselves. The 
lecturer drew attention to the arrangement of the great vessels 
in the reptile, bird, and mammal. He especially directed 
attention to the large number of valves found in the heart of 
the bird, several of these being composed of muscular fibres. 
He believes the complicated and highly differentiated valves 
occurring in the heart of the bird are necessitated by the very 
perfect and elaborate respiratory apparatus; capacious lungs 
and a vigorous circulation being required to maintain the 
temperature and the muscular energy of birds flying and 
diving. He described the circulation as it exists in the 
mammal, and contrasted it with that found in birds. He 
adverted to the pushing and pulling power exerted by the 
heart, the ventricles pushing when the auricles are pulling, 
and vice versci , and gave it as his opinion that, so long as the 
vessels are full of blood, collapse of their walls is impos¬ 
sible, fluids being very slightly compressible. He con¬ 
tended that, under these circumstances, a sucking power 
might be exerted either by the auricles or ventricles. He 
illustrated this by a very simple experiment. He took a 
large vessel with thin walls and fixed an ordinary syringe 
at either end of it. This done, he filled the vessel and one 
of the syringes comfortably with fluid, the other syringe 
being empty. The one syringe, he observed, represented 
the right ventricle, the other the right auricle. When the 
syringe representing the right ventricle was made to dis¬ 
charge its contents into the vessel, that representing the 
right auricle was made to draw into its interior a cor¬ 
responding amount, the fluid being transferred from the 
one syringe to the other without in the slightest degree 
altering the calibre of the vessel. It is simply a question 
of supply and demand, and if the supply equals the 
demand, the vessel may be as thin as tissue paper and 
yet have no tendency to collapse. This is precisely what 
happens in the heart and blood-vessels. The quantity of 
blood in the vessels of the body is nearly always exactly the 
same. The cavities of the heart are of the same size, and 
when the one closes the other opens. The blood ejected by 
the ventricles makes the circle of the lungs and the body 
