This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Now it can scarcely be supposed that the foetal efforts of suction should always be coincident with the maternal act of injection; and if at any time this should not be the case, a fatal accident might happen from the milk being forcibly injected into the larynx. Professor Geoffroy first described the modification by which this purpose is effected; and Mr. Hunter appears to have foreseen the necessity for such a structure, for he has dissected two small foetuses of the Kangaroo for the especial purpose of showing the relation of the larynx to the posterior nares §.
Fig. 418. Foetal Kangaroo.
* P.Z.S., August 1837, p. 348. 1 Memoires du Musee, torn. xxv. p. 48.
2 Trans. Linn. Soc. vol. xvi. p. 61.
§ "See Nos. 3731, 3734, 3735, in the Physiological Series of the Hunterian Museum, in which there are evidence's that Mr. Hunter had anticipated most of the anatomical discoveries which have subsequently been made upon the embryo of the Kangaroo".
The epiglottis and arytenoid cartilages are elongated and approximated, so that the rima glottidis is thus situated at the apex of a cone-shaped larynx (fig. 418, b, a), which projects, as in the Cetacea, into the posterior nares, where it is closely embraced by the muscles of the soft palate. The air-passage is thus completely separated from the fauces, and the injected milk passes in a divided stream, on either side of the larynx, into the oesophagus".
"Thus aided and protected by modifications of structure, both in the system of the mother and in its own, designed with especial reference to each other's peculiar condition, and affording therefore the most irrefragable evidence of Creative foresight, the feeble offspring continues to increase from sustenance exclusively derived from the mother for a period of about eight months. The young Kangaroo may then be seen frequently to protrude its head from the mouth of the pouch, and to crop the grass at the same time that the mother is browsing. Having thus acquired additional strength, it quits the pouch, and hops at first with a feeble and vacillating gait; but continues to return to the pouch for occasional shelter and supplies of food, till it has attained the weight of ten pounds. After this it will occasionally insert its head for the purpose of sucking, notwithstanding another foetus may have been deposited in the pouch; for the latter, as we have seen, attaches itself to a different nipple from the one which had previously been in use".
(2453). Thus therefore are we conducted by the Ovovivipara, as the Marstupialia are properly called, to the most perfect or placental type of the generative system.
(2454). Commencing our account of the reproductive organs of Viviparous Mammalia by examining those of the male sex, we have another striking example of the insufficiency of the nomenclature employed by the anatomist who confines his studies to the human body, when it becomes necessary to describe corresponding organs even in animals organized after the same type.
(2455). True it is that there is the same general arrangement of the generative apparatus; and it is convenient, as far as possible, to apply the same names to structures that apparently represent each other: but a very superficial examination of the facts will serve to show that great differences exist between them; and, accordingly, we are not surprised to find the utmost perplexity and confusion in the descriptions of these parts, arising from the indiscriminate application of the terms employed in human anatomy to totally dissimilar structures.
(2456). It is not, however, our business here to criticize the labours of authors upon this subject; we must content ourselves with selecting an example of one of the more complex forms under which the male genitals present themselves, and leave the reader to contrast the various organs with those met with in the human subject.
(2457). The annexed figure (fig. 419, a) represents the generative viscera of the male Hedgehog. The rectum (a) and the neck of the bladder (h) remain in situ; but the rest of the latter viscus has been removed, and the first portion of the urethra (e) slit open, in order to show the relations of the surrounding parts.
(2458). The testes (b b) present the same structure in all the class, and consist essentially of an immense assemblage of extremely delicate tubuli seminiferi enclosed in a dense albugineous tunic, from which septa pass internally, whereby the seminiferous tubes are divided into several fasciculi: after piercing the proper fibrous tunic of the testes, the sperm-secreting tubes are collected into an extremely tortuous duct, that by its convolutions forms the epididymis, as in Man, and is then continued, under the name of vas deferens, to the commencement of the urethra, into which the two ducts open (b, b b.) In the Horse, and many Ruminants, the vas deferens presents a remarkable structure: before its termination it suddenly swells to a considerable diameter, depending upon the increased thickness of the walls of the canal, which at the same time become cellular, and secrete a gelatinous fluid that escapes into the cavity of the duct.
(2459). In their situation the testes of placental Mammals are found to offer very striking differences. In the Cetacea, the Elephant, and the Seal, they remain permanently in the abdomen, bound down by a process of the peritoneum. In Man, and most quadrupeds, on the contrary, they pass out of the abdominal cavity through the inguinal rings, and are suspended in a scrotal pouch formed by the skin, and a cre-master muscle, and lined by a serous prolongation of the peritoneal sac. The spermatic cords, therefore, formed by the vessels and excretory canal of the testes, will take a different course, in conformity with the variable position of these organs, and, where a scrotum exists, must enter the abdomen through an inguinal canal. Still, from their horizontal posture, quadrupeds are but little liable to hernia, even where the inguinal passages are much more open than in the human subject.