254 Anatomy and Physiology of the 



closed ; but at the time of parturition it is widely dilated, thus 

 forming a free and open passage from the vagina to the interior 

 of the uterus. The coats of the uterus are three, and are united 

 to each other by areolar tissue : the external or serous coat is 

 smooth and continuous with the lining membrane of the abdo- 

 men ; it gives support to the viscus, and by its reflections forms the 

 two broad ligaments. The middle or muscular coat varies con- 

 siderably in strength and thickness, referable to impregnation or 

 non-impregnation ; it is thin and comparatively weak in the latter 

 case, and its fibres, which interlace each other in every direction, 

 become greatly increased during gestation. On the muscular coat 

 the expulsion of the foetus from the uterus in delivery partly 

 depends. The internal or mucous coat has a velvety appearance, 

 and it secretes the menstrual fluid ; but its principal use in the 

 lower animals is to form a bond of connexion between the mother 

 and her young ones, by which their vitality is preserv ed and their 

 development effected. To this we shall hereafter more especially 

 refer. 



Fig. 2. 



C 



d 



a. — The body of the uterus. 



b. — The os uteri. 



c, c. — The horns, one of which is laid open. 



d, d. — The Fallopian tubes, with their fimbriated 



extremities. 



e, e. — The ovaries. 



f,f. — The broad ligaments. 

 g. — The urinary bladder. 

 7t. — The opening of the bladder. 

 i. — The vagina cut open to show the passago 

 leading to the bladder and os uteri. 



We will now add a few remarks on the causes of oestrum, im- 

 pregnation, and the development of the foetus. The term oestrum 



