540 OF SECRETION. 



\ 



elimination take place is often extremely remarkable ; Prussiate of Potash 

 having been detected in the urine within two minutes after it had been intro- 

 duced into the stomach. The variations in this respect would appear to depend 

 chiefly on the degree of concentration of the saline solution, which will affect 

 the rapidity of its absorption, according to the laws of Endosmose ; its recep- 

 tion into the blood being more rapid, in proportion as its density is lower in 

 comparison with that of the circulating fluid. Pure water, or water containing 

 but a small admixture of saline matter, is readily absorbed into the blood-ves- 

 sels of the Intestinal villi ; but it is as readily drawn off through the Kidneys 

 (by the agency, as it would seem, of the Malpighian bodies, 668) ; and con- 

 sequently a large amount may be ingested in a short time. But if the water 

 contain an amount of saline matter equal to that of the Serum, no absorption 

 of it takes place ; it remains in the intestinal tube, and it is voided by the 

 rectum. Further, if the quantity of saline matter in the solution be greater 

 than that of the Serum, not only will no absorption take place, but there will 

 be an endosmose of the water of the blood towards the solution ; so that a large 

 quantity of fluid is discharged by the Intestinal canal. This simple explana- 

 tion, first offered by Liebig,* accounts well for the diuretic effect of most weak 

 saline solutions, and the purgative qualities of stronger ones. For the transit 

 of the peculiar principles of Vegetables, however, it appears that from one to 

 two hours is usually required. The effect of Oil of Turpentine, and probably 

 of other volatile agents, is produced much more rapidly; the characteristic 

 odour of violets being perceptible in the Urine passed but a few minutes after 

 the vapour of the oil had been received into the lungs. 



IV. Mammary Glands. Secretion of Milk. 



683. We now come to those Glands, whose action is rather to elaborate from 

 the Blood certain products, which are destined for special uses in the economy 

 than to eliminate matters, whose retention in the circulating current would be 

 injurious. Pre-eminent amongst these in size and importance, at least during 

 their period of activity, are the Mammary Glands ; which are found only in 

 the animals of the Class Mammalia, and which present themselves in an almost 

 rudimentary state in some of the non-placental subdivisions of the class ( 55). 



a. The structure of the Human Mammary Gland, which has been recently investigated 

 fully by Sir A. Cooper, is very simple, and easily described. It consists of a series of 

 ducts passing inwards from their termination in the nipple, and then ramifying like the 

 roots of a tree, their ultimate subdivisions terminating in minute follicles. The mamillary 

 tubes are usually about ten or twelve in number; they are straight ducts, of somewhat 

 variable size ; and their orifices, which are situated in the centre of the nipple, and are 

 usually concealed by the overlapping of its sides, are narrower than the tubes them- 

 selves. At the base of the nipple, these tubes dilate into reservoirs, which extend beneath 

 the areola and to some distance into the gland, when the breast is in a state of lactation. 

 These are much larger in many of the lower Mammalia than they are in the Human 

 female; their use is to suppjy the immediate wants of the child when it is first applied 

 to the breast, so that it shall not be disappointed, but shall be induced to proceed with 

 sucking until the draught be occasioned ( 426). From each of these reservoirs com- 

 mence five or six main branches of the lactiferous tubes, each of which speedily sub- 

 divides into smaller ones; and these again divaricate, until their size is very much 

 reduced, and their extent greatly increased. The proportional size of the trunk and of 

 its branches appears to follow the same law which governs that of the blood-vessels. 

 The breast is not formed into regular lobes by the ramifications of the ducts ; because 

 they ramify between, and intermix with each other so as to destroy the simplicity and 

 uniformity of their divisions. It is very rarely, however, that they inosculate. The 

 mammary ducts are composed of a fibrous coat lined by a mucous membrane; the latter 

 is highly vascular, and forms a secretion of its own, which sometimes collects in con- 

 siderable quantity when the milk ceases to be produced. 



* Chemistry applied to Agriculture and Physiology, Part ii. 



