306 



MARS U PI ALIA. 



Wagner, (Hecker's Literarische Annalen, Fe- 

 bruarheft, 1834,) but also in works in our own 

 language, as in Hodgkin's Translation of Ed- 

 wards's Influence of Physical Agents on Life, 

 Appendix, p. 438, 1832, have rendered the fact 

 sufficiently familiar. But the connection of 

 this well-known appearance with the mode of 

 formation or multiplication of the blood par- 

 ticles had not before attracted the attention it 

 seemed to deserve; on this subject I have else- 

 where remarked : " In some of the granulated 

 blood-discs of the Perameles the subdivisions 

 producing that appearance were fewer and 

 larger, and were separated by deeper clefts 

 than I had before observed ; they suggested to 

 me the idea that the blood-disc was under- 

 going a spontaneous subdivision into smaller 

 vesicles, and, although my observations are not 

 at present sufficiently numerous to warrant the 

 hypothesis that the development of smaller ve- 

 sicles within itself is a normal property of the 

 ordinary coloured vesicle or blood-disc, yet the 

 obscurity which still hangs over the origin and 

 reproduction of the blood-discs, and the unex- 

 pected constancy of the granulated form in a 

 greater or less proportion of them while recent, 

 and floating in the serum, in different species of 

 animals examined by me, makes me unwilling 

 to suppress any idea naturally arising out of 

 such observations and likely to be suggestive 

 of examination of the same appearances by 

 other microscopical observers."* The general 

 form of the blood-vesicles of the Perameles is 

 the usual circular flattened disc: they presented 

 a greater variety of size than in the Daysurus, 

 but upon the whole a larger average diameter, 

 viz. gj'jgth of an English inch. 



Plialangisla Vulpina. Average diameter of 

 blood-disc jjgjth inch. 



Petaurus sciureus. Ditto ditto ^th inch. 



Macropus penicillatus. Do. do. ^gth inch. 



Macropus major. Ditto ditto 3g ^jth inch. 



Phascolomys Vombatus. Do. do. ^^ih inch. 



The results of the present observations on 

 the blood of the Marsupial quadrupeds cor- 

 respond generally with those obtained from the 

 placental Mammalia, inasmuch as the blood- 

 discs of the species which derives its nutriment 

 from the greatest variety of organized substances, 

 as the Perameles, which subsists on insects, 

 worms, and the farinaceous and succulent ve- 

 getables, are larger than those of the strictly 

 carnivorous Dasyure, and of the herbivorous 

 Kangaroo, the blood-discs of the latter, like 

 those of the placental Ruminant, being the 

 smallest, though not in the same proportion. 

 In each natural group of Marsupialia there is 

 a direct relation between the size of the blood- 

 disc and that of the species. 



Heart. The heart is inclosed in a pericar- 

 dium, and situated in the same relation to the 

 lungs, mediastinum, and thoracic cavity as in 

 the Rodent and most other mammiferous quad- 



* Medical Gazette, 1839, 1. c. My hypothesis 

 has received a certain degree of confirmation by 

 the subsequently published observations on the 

 division of the' corpuscles of the blood by Mr. 

 Quekrtt, (Med. Gazette, January, 1840.) and by 

 Dr. Martin Barry, Philos. Trans. 1840, p. 595. 



rupeds. It offers no peculiarity in its general 

 outward form. The apex is less obtuse in 

 some species, as the Phalanger and Wombat, 

 than in others, as the Kangaroo. The serous 

 layer of the pericardium is reflected upon the 

 large vessels near to the heart. The fibrous 

 layer of the pericardium adheres to the sternum 

 in the Kangaroo. The appendix of the right 

 auricle is always divided into two angular pro- 

 cesses, (a, a,Jgs. 131 and 132,) one in front 

 and the other behind the trunk of the aorta. 



Fig. 131. 



Heart of the Kangaroo. 



Besides this characteristic modification of its 

 external form, the right auricle presents some 

 still more essentially marsupial conditions 

 in its interior. There is no trace, for example, 

 of a ' fossa ovalis' or an ' annulus ovalis' in 

 any marsupial animal;* and the absence of 

 these structures, which are present in the heart 

 of all the placental Mammalia, doubtless re- 

 lates to the very brief period during which the 

 auricles intercommunicate in the Marsupials, 

 and to the minute size, and in other respects 

 incompletely developed state, at which the 

 young marsupial animal respires air by the 

 luno-s, and has the mature condition of the 

 pulmonary circulation established. The right 

 and left auricles intercommunicate by an 

 oblique fissure in the uterine embryo of the 

 Kangaroo, when two-thirds of the period of 

 gestation is past, but every trace of this fcetal 

 structure is obliterated in the subsequent growth 

 of the heart; so that in the mature animal the 

 wide terminal orifice of the posterior cava is 



* Physiological Catalogue, Mus. Royal College 

 of Surgeons, 4to. vol. ii. p. 52. 



