IN THE VENTRICLES OE THE VERTEBRATE HEART. 
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nicating with the ventricular cavity, and conducting to canals of various sizes. These 
canals freely unite with each other, and, as they penetrate the ventricular wall only to 
certain depths, may on this account be denominated terminal canals. They serve to 
increase the size of the ventricular cavity, and render the ventricle lighter than it 
would otherwise be. 
The ventricle of the fish may be regarded as a conical-shaped muscular bag; the fibres 
of which are curiously interwoven to secure the greatest amount of strength with the 
least possible material, and, what is not less desirable in a physiological point of view, 
to ensure that the organ shall contract in all directions, the more thoroughly to eject 
the blood from its interior. There is, however, in the ventricle under consideration, as 
far as I have been able to discover, no principle in the arrangement similar to that 
which, as I shall endeavour to explain, occurs in the ventricles of the higher vertebrata. 
VENTRICLE OE THE REPTILE. 
The form of the ventricle of the reptile’s heart is intermediate between the well- 
defined pyramidal shape in the fish, and the finely rounded conical form of the ven- 
tricles in the bird and mammal. Thus it has the dorsal surface flattened, as in the fish, 
while the two anterior surfaces present a somewhat convex outline. 
The arrangement of the fibres in the ventricle of the reptile is so similar in many 
respects to that met with in the ventricle of the fish, that a separate description appears 
unnecessary. There are, however, points of difference deserving of notice. The fibres 
composing the external and internal layers in the ventricle of the reptile are more 
decidedly vertical than in that of the fish, and run with fewer interruptions from the 
base to the apex, and from the apex to the base. This difference is well seen in the 
ventricle of the python and alligator, and is an approach to what is found in similar 
layers in the ventricles of the bird and mammal. The external layer in the ventricle 
of the reptile is thinner than in the ventricle of the fish, the internal layer being com- 
paratively much thicker. The transverse layer, in the ventricle of the reptile, is also 
thinner than in the ventricle of the fish, the perforating fibres which run between the 
external and internal layers being on the contrary increased both in number and size. 
The perforating fibres have further a tendency to split up and give offsets to run athwart 
the ventricle in the direction of the transverse layer, and in this manner supply the 
deficiency in its thickness. The ventricular cavity of the reptile is smaller than in the 
fish, while the number of terminal canals which proceed from it to ramify in the ventri- 
cular wall is greater. This circumstance renders the wall of the ventricle of the reptile 
at once thicker and less dense than that of the fish. Lastly, the ventricular cavity 
of the reptile is variously shaped, according as the septum is absent or present and 
partially or fully developed. 
VENTRICLES OF THE BIRD. 
The ventricles of the bird so closely resemble those of the mammal in appearance 
and structure, that one description will suffice for both. Care however will be taken, 
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