150 PRINCIPLES OP VETERINARY SCIENCE 



and forms the posterior opening of the genital canal. It is 

 about 5 inches long. At its margin the mucous membrane, 

 which lines the entire genital tract, meets the skin. On the 

 floor of the vulva may be seen the external orifice of the urethra. 

 The clitoris, a small erectile body, is lodged within the inferior 

 commissure of the vulva. 



The mammary glands are two in number and placed side by 

 side on the middle line of the abdominal wall between the thighs. 

 The teat or nipple in the mare is perforated by two or three ori- 

 fices for the passage of the milk. The glands are composed of 

 secreting cells which line minute acini that connect with small 

 ducts. These ducts unite to form a number of tubes which drain 

 into a central sinus or milk cistern. The function of the mam- 

 mary glands is to secrete milk upon which the foal subsists 

 for some months after birth. In the cow these glands are com- 

 monly spoken of as the udder (Fig. 51.) This organ is often very 

 large and pendulous in dairy breeds. Usually it is considered as 

 consisting of two glands divisible into four quarters. Although 

 each gland is separated by a septum, there is no visible division 

 between the two quarters of the same side. However, injections 

 of fluids of different colors into the two teats of the same gland 

 demonstrate that the cavities drained by them do not communi- 

 cate. The cavity in the center and above each teat is known as 

 the milk sinus. 



THE PHYSIOLOGY OF REPRODUCTION 



Reproduction may be defined as the power to produce new but 

 similar individuals. It requires the union of the spermatozoon 

 from the male and the ovum from the female. The female is 

 concerned with the complete reproduction process which consists 

 of the following stages: copulation, impregnation, gestation, 

 parturition, and lactation. The male is concerned only with 

 the first two of these stages. 



Puberty is the earliest age at which animals are able to pro- 

 create or beget. Animals of both sexes pass through this stage 

 before they are sexually mature. It varies with species, breed, 

 nourishment, and precocity of the individual. In highly fed 

 animals and those with a nervous temperament it appears ear- 

 liest. Too early stud service may retard the development of the 

 stallion. He may be used on a few mares when two years of 



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