OCR Text |
Show 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 diaphragm 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 heait and diaphragm. 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 suspended 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 support of the liver and other alimentary viscera. The envelopment of the heart by the liver in the Ostrich possibly affords an illustration 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 extension 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 " (Marshall): from which fact it follows that the part the diaphragm plays in 22* 1 9 0 3 . ] TRANSPOSITION OF MAMMALIAN TESTES. 3 3 7 |