HISTOLOGY. 207 



and thus the secretion is passed towards the main 

 ducts, which serve to store and transmit what they 

 receive. There is this peculiarity about the lactif- 

 erous ducts : they are not strictly uniform in size 

 throughout, nor do they lessen or increase in size by 

 regular gradation, but, slightly contracted at their 

 inlet and outlet, have a bulge between, thus, in 

 form, a series of saccular cavities. The ducts and 

 reservoirs are thus, in one sense, the same. The 

 constricted portion of the reservoirs is formed of 

 elastic tissue, which underlies the mucous mem- 

 brane of the ducts. Thus, by the retardation of the 

 milk, as it passes from the vesicles, where it is man- 

 ufactured, towards the teat, its outlet, the pressure 

 of the accumulated quantity is equalized to a cer- 

 tain extent throughout the gland. These ducts and 

 pouches are lined by a vascular mucous membrane. 

 On the exposed surface of the mucous membrane is 

 a thin covering of tessellated epithelium to defend it 

 from injury. As we reach the coecal extremity of 

 the system we are describing, we observe the epi- 

 thelium changing its character on the edge of the 

 glandules or vesicles. The vesicle itself is lined 

 with cells, which differ in size. These cells are the 

 secreting portion of the gland, and by their own 

 increase and casting off are themselves the morpho- 

 logical portion of the milk, the fat globule. These 

 acini are surrounded by a net-work of capillaries, 

 which form a rete or net on their surface, and furnish 

 the blood for the use of these organs. Under the 

 stimulus of abundant supply of force from this blood, 



