File:Image from page 499 of "Comparative anatomy of vertebrates" (1907) (20676852801).jpg

Title: Comparative anatomy of vertebrates Identifier: comparativeanato00wied Year: 1907 (1900s) Authors: Wiedersheim, Robert, 1848-1923; Parker, W. N. (William Newton), d. 1923 Subjects: Anatomy, Comparative; Vertebrates Publisher: London, Macmillan and co. , limited Contributing Library: MBLWHOI Library Digitizing Sponsor: MBLWHOI Library

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Text Appearing Before Image: 482 COMPARATIVE ANATOMY Marsupials1 (p. 35): in the former this serves to protect the egg, and in the latter the young, which are born in a very unripe con- dition, thus rendering possible a longer connection between the mother and embryo during lactation. The aperture of the marsu- pial pouch opens anteriorly or posteriorly, according to the mode of life of the animal, and is provided with a sphincter muscle capable of closing it. In Marsupial embryos the margins of the lips become partially fused secondarily and temporarily to form a suctorial mouth, by means of which the young, many of the organs of which are still in a " larval " condition, become attached to the teats (cf. p. 438). In male Mammals, the testes arise in the same relative position as the ovaries of the female. The ovary, however, does not become shifted further backwards than the pelvis in the course of development; but the testis may pass out of the abdomen through an inguinal canal into a purse-like outgrowth of the integument in Testis Mesorchii'ni Vas deferens Peritoneum Int. obi. m. Transversalis Ext. olil.

Text Appearing After Image: Cremaster-sac Gubeniacuhim (Lig. scroti) Area scroti FIG. 362.—DIAGRAM OF THE PARTS CONCERNED IN THE DESCENT OF THE TESTIS. (After M. Weber.) a, ligamentum testis ; b, ligamentum inguinale ; c, muscular conus inguinalis. the inguinal region called the scrotal sac, which is lined by a continuation of the peritoneum, the tunica vaginalisY$36'2). The two scrotal sacs may remain separate, or unite to form a scrotum : in Marsupials this is situated in front of, and in placental Mammals behind the penis. If the inguinal canals remain widely open, the testes may be withdrawn periodically into the abdomen (as e.g. in Rodentia and Insectivora, in which they only descend at sexual maturity) : this is effected by means of the crcmastcr muscle, a more suitable name for which would be the levator s. retractor testis. This muscle is a continuation of the fibres of the internal oblique and transversalis, or of the latter only, and corresponds to the " compressor mammae " of female Marsupials. When the inguinal canals become reduced (as e.g. in Man) the testes remain perma- nently in the scrotum. In many Mammals, however (e.g. Mono- 1 Traces of the marsupial folds occur in young stages of various male Marsupials, and indications of a Marsupial apparatus have been described in several of the higher Mammals.

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