EXTERNAL OKGAXS OF GEXEILITIUX. 88 



somewhat independent of each other ; so that one or more may be 

 diseased or deranged in function, without the otiiers being involved. 

 Externally, this envelope is closely adherent to tlie skin, through the 

 medium of a thin but dense layer of connective tissue. 



The (jlctiuhdar tissue offers the same arrangement as other conglome- 

 rate glands, and is composed of acini or caical vesicles clustered, like 

 grapes on their stalk, around the tuhiili lactifcri, or ultimate terminations 

 of the excretory ducts. These, commencing by cul-de-sac extremities, 

 open into one another to form dilatations {ampnlUe), and finally con- 

 verge into a number of principal canals, which end in the galacto- 

 phorous sinuses (sinus lactei). The acini of the lobules, as well as the 

 ducts, are lined by polyhedral epithelium ; this becomes spherical and 

 infiltrated with fat during lactation. 



The galactopliorous sinuses or reservoirs are situated slightly above 

 the base of the teat, and are generally two in number — one in front, 

 the other behind ; though there are sometimes three, and even four. 

 They nearly always comnmnicate with each other, and are prolonged 

 into the teat by a corresponding number of terminal and independent 

 excretory canals, whose orifices are always very narrow, and are seen 

 at the free extremity of that body, which is obtuse and rounded. 

 Collectively, these excretory canals are much wider at tiie base of the 

 teat than at the extremity ; the orifices are usually behind each other, 

 and are about a line apart ; and the canals and orifices are lined by 

 a fine membrane which is continuous with the skin, the latter being 

 closely adherent at this part. 



The length of the teats varies with use ; the elastic or dartoid tissues 

 surrounding them, composed of non-striated circular and longitudinal 

 fibres, renders them capable of a kind of erection, under the influence 

 of stimuli. The extremity of the teat is well provided with this tissue, 

 which acts as a sphincter, and prevents the passive flow of the milk. 



Connective tissue, bloodvessels, nei"ves, and absorbents complete the 

 organization of the mamma?. The arteries are given oft' from the 

 external pudic ; the veins are of two orders — deep, which follow the 

 arteries, and superficial. The nerves are derived from the renal or 

 mesenteric plexus. 



As before mentioned, the mamma> undergo remarkable modifications 

 at the age of puberty and the termination of gestation ; and these 

 changes have reference not only to their volume and secretion, but also 

 to their minute structure. In the young or virgin Mare, they are hard, 

 and can scarcely be perceived ; and their dimensions are not nmch 

 increased in those which have had only one or two foals, though the 

 teats are usually larger than before ; but when they have borne several 

 foals, the mamnue continue somewhat enlarged and pendulous. When 

 gestation is not going on, the glandular cul-de-sacs are contracted and 

 wasted-looking; tlie lining membrane is shrivelled on itself, and covered 

 with only a polygonal epithelium. .\t the termination of gestation, 

 however, the mannna; assume the functions of the uterus to a certain 

 extent ; the vesicles become enlarged, and new ones are developed ; the 

 epithelium presents a spherical shape, is charged with fat granules, and 

 fills the acini ; the entire gland has become progressively, but greatly 

 increased in si>:e, and instead of being soft to the touch it now feels 

 firm. Shortly before parturition the secretion of milk commences, and 

 soon after that event the glandular cavities become fully distended, and 



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