of the Mammary Organs of the Kangaroo. 7t 



the pouch in the living animal, that it is produced at will by the 

 mother, apparently for the purpose of preparing the parts for 

 the more ready transmission of milk to the young. On these 

 occasions the gland and teat will be found of nearly double the 

 size which a full injectment of the ducts is capable of producing. 

 The cause of this additional enlargement however was, as I 

 conceive, explained by an examination of the vascular system in 

 the parts alluded to ; for by continuing the dissection, and care- 

 fully removing the compressing muscles of the gland and teat, 

 it was found that, from the number and size of the veins, any 

 obstruction to the circulation of blood through these vessels 

 would in itself be sufficient to occasion a degree of distention by 

 which the extraordinary increase of size in these parts might be 

 adequately accounted for. The distribution of the veins in the 

 marsupial gland is not characterized by any remarkable appear- 

 ance as regards their course ; from their immense number, 

 however, the whole surface of the gland presents when closely 

 examined a reticulated congeries of vessels, which, with those of 

 the interior, unite into larger trunks, the greater number of which 

 terminate on the outer convex surface of the gland in a single 

 vessel {tab. S. f. 2. a.), which passes first between the two layers 

 of the proper muscle of the mammae, afterwards descends upon 

 the tendon of the external oblique, and ultimately empties itself 

 into the external iliac vein. This vessel, however, conveys only 

 a part of the blood from the marsupial gland towards the heart, 

 since another venous trunk, issuing from the inner margin of the 

 gland, descends in a similar manner to the same termination, 

 namely, to the iliac vessels. This vein conveys not only the 

 remaining portion of blood from the gland, but also the whole 

 of that which returns from the marsupial teat (tab. S.f. 2. b.). 



Venous circulation, carried on through vessels thus situated, 

 must necessarily meet with obstruction from the action of the 



compressing 



