. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. well-developed penes, resembling those of crocodiles. The penis in all mammals, with the exception of monotremes (Fig. 398b), is a closed tube and not a grooved structure as in reptiles. In- termediate forms between reptiles and mammals are not known. It is typically a turgescent organ under the control of vaso-dilator nerves which arise as autonomic fibers in the sacral region of the spinal cord, and is composed of two masses of erectile tissue side by si
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. well-developed penes, resembling those of crocodiles. The penis in all mammals, with the exception of monotremes (Fig. 398b), is a closed tube and not a grooved structure as in reptiles. In- termediate forms between reptiles and mammals are not known. It is typically a turgescent organ under the control of vaso-dilator nerves which arise as autonomic fibers in the sacral region of the spinal cord, and is composed of two masses of erectile tissue side by side, the corpora caver- nosa, with a third similar mass, the corpus spongiosum beneath them, held together by fibrous tissue and enveloped by a layer of loose skin (Fig. 399). The corpus spongiosum is perforated for its entire length by the urethra. Consequently the urethra is considerably longer in the male than in the corresponding female. It terminates in an en- largement of the corpus spongiosum called the glans (Fig. 398d), which is split in marsupials where the female has a double vagina. In some animals, for example the cat, guinea pig, and wombat, the glans is beset with horny recurrent spines or corneal scales that stimulate the female during copulation, although in most cases the skin extending over the surface of the glans is extremely sensitive and delicate without any hard corneal layer. Ruminants, cetaceans, and some rodents are exceptional in that there is no glans present, while sheep, goats and many antelopes possess a peculiar threadlike elongation of the sperm duct beyond the end of the penis itself, called the processus urethralis (Fig. 400). Among higher animals the glans is enveloped in a double fold of retrac- tile skin, the preputium (Figs. 384 and 398d). This is present only in forms with a pendent penis and is homologous with the sheath in which the penis is withdrawn in many animals such as Fig. 400. a, distal end of ram's penis, showing gl
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