Uterus

The uterus is located inside the pelvis immediately dorsal (and usually somewhat rostral) to the urinary bladder and ventral to the rectum. 

The uterus can be divided anatomically into four segments: The fundus, corpus, cervix and the internal os.

The three layers, from innermost to outermost, are as follows:

Endometrium
The lining of the uterine cavity is called the "endometrium". It consists of the functional endometrium and the basal endometrium from which the former arises. Damage to the basal endometrium results in adhesion formation and/or fibrosis (Asherman's syndrome). In all placental mammals, including humans, the endometrium builds a lining periodically which is shed or reabsorbed if no pregnancy occurs. Shedding of the functional endometrial lining is responsible for menstrual bleeding (known colloquially as a "period" in humans, with a cycle of approximately 28 days, +/-7 days of flow and +/-21 days of progression) throughout the fertile years of a female and for some time beyond. Depending on the species and attributes of physical and psychological health, weight, environmental factors of circadian rhythm, photoperiodism (the physiological reaction of organisms to the length of day or night), the effect of menstrual cycles to the reproductive function of the uterus is subject to hormone production, cell regeneration and other biological activities. The menstrual cycles may vary from a few days to six months, but can vary widely even in the same individual, often stopping for several cycles before resuming. Marsupials and monotremes do not have menstruation.

Myometrium
The uterus mostly consists of smooth muscle, known as "myometrium." The innermost layer of myometrium is known as the junctional zone, which becomes thickened in adenomyosis.

Perimetrium
The loose connective tissue around the uterus.