THE UROGENITAL SYSTEM 107 



passed. The os uteri projects freely into the forepart of the vaginal 

 cavity. 



The vulva is the passage that continues the vagina backward, 

 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 is perforated by two or three orifices 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 mammary glands is to 

 secrete milk upon which the foal subsists for some months after 

 birth. 



That there is an intimate relationship between the activity of the mammary 

 glands and the generative organs of the female there can be little doubt. 

 Starhng injected an extract of fetal tissues into the body of a virgin. This 

 caused the mammary gland to become active and secretion of milk was actually 

 produced. The experiment shows that in the fetus there is some substance, or 

 property, which has a direct action upon the mammary tissue. 



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 stallion and the ovum from the mare. Before young horses 

 can reproduce they must attain sexual maturity or puberty. Too 

 early stud service may retard the development of the colt. He may 

 be used on a few mares when two or three years of age, but should 

 not be placed in regular service until four years old. The filly 

 is usually sexually mature when one year old, but should not be- 

 come a mother until she reaches the age of three years, or four if 

 she is not precocious. 



Heat, rut, estrum are expressions commonly applied to the 

 period during which the mare exhibits a desire to mate. Mat- 

 ing will be permitted only during the period of estrum. This 

 should occur between the fourth and the twelfth day after she 



