452 
DR. PETTIGREW ON THE ARRANGEMENT OF THE FIBRES 
IV. The fibres composing the external and the internal layers are found at different 
depths from the surface, and from the fact of their pursuing opposite courses cross each 
other, — the fibres of the first external and last internal layers crossing with a slight 
d eviation from the vertical, as in the letter X ; the succeeding external and internal 
layers, until the fourth or central layer, which is transverse, is reached, crossing at 
s uccessively wider angles, as may be represented by an placed horizontally : — 
V. The fibres composing corresponding external and internal layers, such as layers 
one and seven, two and six, &c., are continuous in the left ventricle at the left apex, and 
in the right ventricle in the track for the anterior coronary artery, the fibres of both 
ventricles being for the most part continuous likewise at the base *. 
VI. From this distribution of the fibres, it follows that the first and seventh layers 
embrace in their convolutions those immediately beneath them, while these in turn 
embrace those next in succession, and so on until the central layer is reached — an 
arrangement which may in part explain, alike, the rolling movements and powerful 
action of the ventricles. 
VII. The fibres of the right and left ventricles anteriorly and septally are to a certain 
extent independent of each other ; whereas posteriorly many of them are common to 
both ventricles ; i. e. the fibres pass from the one ventricle to the other, — an arrange- 
ment which induced Winslow f to regard the heart as composed of two muscles enve- 
loped in a third. It will be evident from this distribution of the fibres, that while the 
ventricles are for obvious reasons intimately united, they nevertheless admit of being 
readily separated. 
VIII. If the hinge-like mass of fibres (common fibres) which unite the right ventricle 
to the left posteriorly be cut through, and the right ventricle with its portion of the 
septum detached, the left ventricle will be found to be nearly as complete as it was 
before the separation took place, and to consist of four sets of conical spiral fibres — two 
external and two internal sets. 
On the other hand the right ventricle, and its share of the septum, consists only of 
conical-shaped spiral fragments of fibres, or at most of flattened rings — a circumstance 
which, when taken in connexion with others to be mentioned presently, has induced me 
* The late Dr. Duncan, jun., of Edinburgh, was aware of the fibres forming loops at the base, but seems to 
have had no knowledge of the continuity being occasioned by the union of the fibres of corresponding external 
and internal layers, or that these basal loops were prolongations of like loops formed by similar corresponding 
external and internal layers at the apex — a view which the author believes is here set forth for the first time, 
t Memoires de l’Academie Royale des Sciences, 1711, p. 197. 
