This, the smaller of the three bodies composing the penis (g, fig. 226), is situated in a groove which runs along the lower border of the corpora cavernosa. Behind, it commences at the perineum in an enlargement termed the " bulb ", and terminates in front in an expanded free extremity, which during erection resembles the rose of a watering-can. This is the "glans penis".

The corpus spongiosum is composed of a loose erectile tissue, similar in appearance to that of the corpora cavernosa, but of much finer texture. The urethral canal, by which the urine is conveyed from the bladder, is surrounded by it for the greater part of its length, and a thin muscle (accelerator urinee) encloses the whole from the ischial arch behind as far forward as the glans.

This muscle, by compressing the urethra from behind forward, expels the last few drops of urine after the efforts of the bladder have ceased.

The Prepuce (l, fig. 226), commonly spoken of as the " sheath ", is a double layer of loose skin folded inward to form a recess in which the free portion of the penis is enclosed.

The inner fold of the prepuce, although continuous with the outer skin, partakes of the character of mucous membrane. It is thin, supple, vascular, and freely lubricated with an unctuous greasy matter, secreted by numerous sebaceous glands which exist in and beneath it. It is this substance which, on becoming inspissated and dry, forms the dark gray masses which accumulate in the scrotum, and sometimes provoke irritation and disease. During erection this inner layer is effaced, and forms a covering to the body of the organ as it protrudes from the prepuce. The sheath is supported by an elastic band - "suspensory ligament" of the sheath - reflected from the under surface of the abdomen.

The loud gurgling sound emitted by some horses when in motion is due to the sudden displacement of air by the to-and-fro movements of the penis within the sheath.