1903.] TRANSPOSITION OF MAMMALIAN TESTES. 337 
and liver. It is a fibrous membrane, concave forwards, with a 
muscular attachment at either side to the ribs and intercostal 
tissues, which it joins in about the middle of their course...... 
The pleural cavity is closed above and below by the fibrous dia- 
phragm becoming blended with the first and last ribs. The 
anterior thoracic cavity, which contains the pericardium-coated 
heart in its upper part, entirely independent of the pleural 
cavity, is divided into two by a dense fibrous membrane 
which springs from two vertebral crura, much as the human 
diaphragm, and extends above the line to join the sternum along 
the border which articulates with the ribs, leaving the heart entirely 
in front of it; its concavity is directed downwards and forwards, 
and it is separated from the diaphragm proper by very large air-cells, 
The liver is completely separated from the abdominal cavity by a 
fibrous membrane, so that when the included viscera are removed, 
it is not at all brought into view. The mesentery is very dense 
and strong” (Garrod & Darwin). A comparison of the struthious 
and mammalian diaphragms affords further confirmation. In the 
Ostrich, owing to the anomalous position of the avian lungs (their 
close application to the costal skeleton ensuring the intactness of 
the air-cell connections), the liver has practically assumed their 
role, as regards conformation, in relation to the heart and dia- 
phragm. Hence the diaphragm is concave anteriorly, consequent 
on the necessity for the retention of the liver. In the mammal, 
on the other hand, the lungs are of large volume and freely sus- 
pended in the ventral portion of the thorax, enveloping the heart. 
If the liver were anterior to the diaphragm it would, as before 
remarked, during locomotion periodically exert great pressure on 
the lungs ; hence the diaphragm here becomes the forward sup- 
port of the liver and other alimentary viscera. The envelopment 
of the heart by the liver in the Ostrich possibly affords an illus- 
tration of that alternative to the formation of a diaphragm above 
referred to. 
From these and other considerations, we can more precisely 
interpret the anterior convexity of the diaphragm, which is due 
to two causes: the forward pressure in the median line of the 
large abdominal viscera, and the backward lateral extension of 
the capacious lungs. Evidence as to the cooperation of this last 
factor is afforded by such cases as that of the Manatee, in which, 
on account of its subaqueous habits, unusually developed organs 
of respiration are required, it also being essential that their ex- 
tension should be dorsal in order to ensure the ventral situation 
of the centre of gravity of the body. Here, as Mivart remarks, 
the diaphragm is so oblique that the thorax ‘“ extends backwards 
above the whole length of the abdominal viscera”; and a similar 
conformation is found in Cetacea and perhaps other groups. In 
development, the diaphragm is ‘formed from a couple of septa, 
dorsal and ventral respectively, which arise independently, and 
are for some time quite distinct from each other” (Jarshall) : 
from which fact it follows that the part the diaphragm plays in 
20% 
