POLYGASTRIA. 



ther in front of the heart ; but in the adult 

 human subject this is not the case, the pericar- 

 dium coming into immediate contact with the 

 anterior thoracic pahetes. Also the pleurae are 

 prevented by the adhesion of the pericardium 

 to the diaphragm from adhering to one another 

 below the heart. It is almost superfluous to 

 state that the heart and pericardium encroach 

 more upon the left pleural cavity than upon the 

 right. The median thoracic septum of the hu- 

 man subject is able, partly on account of its 

 small antero-posterior extent, to resist any con- 

 siderable lateral displacement, such as might 

 result from accumulation of effusion into one 

 pleural cavity ; but in deep-chested animals it 

 admits of displacement to such an extent that 

 the whole of the chest may be filled with an 

 effusion into one pleural cavity. In a Chetah, 

 which died of pleurisy at the gardens of the 

 Zoological Society, dissected by the author, the 

 immediate cause of death was suffocation occa- 

 sioned by effusion into the right pleura, which 

 occupied the whole chest, and compressed the 

 left lung, the left pleura being unaffected. The 

 mediastinum may be regarded as a kind of 

 mesentery to the heart, and in some reptiles it 

 is very obviously seen to be a part of the great 

 median mesentery wherein all the viscera are 

 suspended. This great median mesentery of 

 reptiles is attached to parietes in front as well 

 as behind as far down as the falciform liga- 

 ment of the liver; as though a fold of serous 

 membrane had been pulled down by the umbi- 

 lical vein. In some reptiles, as the chame- 

 leon, the anterior parietal attachment is conti- 

 nued even down to the small intestines, so that 

 the stomach and part of the small intestine are 

 enclosed between the layers of the mediasti- 

 num. The serial homology of this septum is ob- 

 scured in Mammalia by the diaphragm being 

 interposed between it and the other mesenteries. 



The only fold or duplicature made by each 

 pleura that is comparable to the mesenteries or 

 omenta formed by the peritoneum is that called 

 the broad ligament of the lung ( ligamentum 

 lutum pulnionix). It is a fold carried down- 

 wards and backwards from the root of the lung. 

 It may be described as having four edges, the 

 upper one of which is attached along the lower 

 aspect of the root of the lung; the outer one 

 is attached to the inner aspect of the lung from 

 its root to its lower border; the inner one is 

 attached to the mediastinum from the root of 

 the lung downwards and backwards to the 

 cesophageal opening in the diaphragm ; the 

 remaining edge is free and directed outwards, 

 downwards, and backwards. Its inner or 

 mediastinal attachment is by far the longest, so 

 that its figure is four-sided, with one corner 

 extremely drawn out or prolonged. 



There are frequently found, especially about 

 the pericardium, numerous pyriform masses 

 of fat covered with pleura, like appendices 

 epiploicffi. 



The outer surface of the pleura is but loosely 

 adherent to the ribs and intercostal muscles; it 

 is more firmly connected with the diaphragm 

 and pericardium, and still more firmly with 

 the lungs. The adhesion of the two pleura 



in the mediastinum is extremely loose in the 

 human subject, large quantities of areolar tissue 

 and frequently fat being interposed ; so that in 

 many subjects they can hardly be said to come 

 into immediate contact at all. 



The pleura covering the ribs and that forming 

 the mediastinum is strengthened by a fibrous 

 layer, but that covering the lungs is destitute 

 of such fibres, and consequently extremely thin 

 and delicate. The terms pleura pnlmonulis, 

 pleura costalis, and pleura diaphragmatica, are 

 applied respectively to those parts of the pleurae 

 which are connected with the lungs, the ribs, 

 and the diaphragm : and these expressions are 

 frequently found extremely convenient. 



From the extreme frequency of abnormal 

 adhesions of the apposed surfaces of the pleura 

 it appears that this serous membrane is un- 

 usually liable to inflammation, which liability 

 may be due to its being unusually exposed to 

 external circumstances through its extreme 

 proximity to the air in the lungs. 



The pleura is peculiar to the class Mammalia. 

 In Birds the lungs are adherent to the thoracic 

 parietes; and in Reptiles, there being no dis- 

 tinction of thorax and abdomen, they are in- 

 vested by the peritoneum. To this, however, 

 there is an exception in the Crocodilians, in 

 which reptiles a rudimentary diaphragm exists. 

 The pleurae of these animals are disposed 

 around the outer, anterior, and posterior, but 

 not the inner aspect of each lung; so that the 

 lung seems to be adherent to the mediastinum. 

 (Simon Rood Plttard.) 



POLYGASTRIA. A name applied by 

 Professor Ehrenberg, of Berlin, to an immense 

 class of microscopic animalcules which exist in 

 countless millions in water of various kinds, 

 both salt and fresh, more especially in such 

 as contains decomposing animal or vegetable 

 substances. 



Many forms of these beings are indescribably 

 minute, some of them measuring not more 

 than the 32,000th part of an inch in length, 

 and all of them are of such tiny dimensions as 

 to require the utmost penetration of the micro- 

 scope and the most patient industry on the part 

 of the observer to make out their organization. 

 A few of the largest are, indeed, barely distin- 

 guishable by the unassisted eye; but, generally 

 speaking, they are quite invisible; and had it 

 not been for the invention of the microscope, 

 we should, even at this day, have been ignorant 

 of their existence. 



The numbers in which these creatures abound 

 baffles all expression. It has been ascertained, 

 and the fact may easily be proved with a good 

 microscope, that, possessing the dimensions 

 above referred to, say the 24,000th part of an 

 inch, many of these living atoms crowd the 

 water in which they are found to such an extent 

 that they are not separated from each other by 

 a space greater than the size of their whole 

 bodies; so that by a very little calculation it 

 will be seen that a single drop of such water 

 contains more of these active existences than 

 there are human beings upon the surface of this 

 globe. And when the mind reflects upon their 



