EXTERNAL GENERATIVE ORGANS. ^^Z 



orifices are usually behind each other, and are about aline 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, blood-vessels, nerves, and absorbents complete the 

 organization of the mammae. The arteries are given off from the external 

 pudic ; the veifis 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 mammae 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 much 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 mammae 

 continue somewhat enlarged and pendulous. When gestation is not 

 going on, the glandular cul-de-sacs are contracted and wasted-looking ; 

 the lining membrane is shrivelled on itself, and covered with only a 

 polygonal epithelium. At the termination of gestation, however, the 

 mammae 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 size, 

 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 assume their maximum 

 dimensions, which are maintained, with slight variations, during the entire 

 period of lactation. When this period is terminated, the secretion 

 gradually ceases^ and the gland again assumes its quiescent condition, 

 and nearly its ordinary size.* 



Differences in other Animals 



Cow. 



In the Cow the mammae are also inguinal, as in the Mare, and each lat- 

 eral mass, although enveloped in a single fibrous capsule, is made up of 

 two quite distinct glands — or '" quarters," as the3^ are commonly termed — 

 and which can be seen, or felt as limited by a slight depression. Each 

 gland has its corresponding teat, much more developed than that of the 

 Mare ; so that this animal really possesses four mammae and four teats. 

 The glands are compacted into a roundish mass, which is more or less 

 pendulous when they are in active function ; in the centre of each, and 

 at the base of the teat, there is a single large galactophorous sinus, which 

 is the general confluent of all the lactiferous ducts, and which opens ex- 

 ternally through the teat by a single excretory canal. This canal is widest 



* In the male we find rudimentary teats, which are concealed within the prominent annular fold of 

 integument towards the extremity of the prepuce. They are not always present, however ; though they 

 are so in the Ass, which has them largely developed. 



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