relatively smaller in the male Echidna than in the Ornithorhynchus. The gland is situated lower down, in the popliteal region, between the insertions of the deep-seated fasciculi of the adductor femoris and the origins of the gastrocnemius; it is of subspherical form, about the size of a pea, with a smooth exterior; the excretory duct, wide at the commencement, soon contracts into a filamentary canal, which again enlarges to form a small reservoir for the secretion just above the base of the spur. The duct is accompanied and partly covered by the posterior tibial nerve.

The spur is a round, curved, sharp-pointed cone, traversed by a canal, continued from the reservoir, and opening on the convex side of the spur a little way below the pointed extremity.

The true nature and use of this apparatus has not yet been determined. Its close analogy with the poison-apparatus in other animals suggests a corresponding function; but no well authenticated case of symptoms of poisoning consequent upon a wound inflicted by the spur has been recorded: it seems on the contrary that the Ornithorhynchus possesses not the instinct of availing itself, when attacked or annoyed, of a weapon so formidable as, upon this theory, the spur must be.1

1 CCXXXIV". p. 236.

CHAPTER XXXVII.

GENERATIVE ORGANS OF MAMMALIA.

OUTWARD characters of sex are least marked in Lissencephala. To distinguish the male from the female Mole, Shrew, Hedgehog, Sloth, Rodent, requires close scrutiny, if not dissection. The male Monotreme is known by his heel-spur; the female Marsupial by her pouch and by her smaller (in Kangaroos much smaller) size. Among Cetacea the tusk distinguishes the male Narwhal, and the larger head the male Cachalot: in Seals the canines are usually larger in the male. External parts of generation are conspicuous in other Gyrencephala. Besides these, most Ruminants have sexual characters in the horns, either by their presence or greater size; the Stallion and Boar have the tusks: these by their greater length distinguish the male Elephant, especially the Indian kind. In the Carnivora the male is the strongest: the Lion is dignified by his grand mane. The larger canines, with greater general size, mark the male sex in most Quadrumuna up to and including the Gorilla. Besides some differences in size and proportions of body, developments of hair are the outward marks of sex in Bimana.

A. MALE ORGANS.

In the Mammalian class the testes attain their most compact form, with most definiteness and finish of parts, in unravelling which anatomy has surpassed itself, chiefly upon the glands as they exist in Man, from which type of testicular structure there is no essential departure in the lower orders. The peritoneum adds a serous layer to the proper sclerous covering of the gland: and when this passes, as in the majority of Mammals, out of the abdomen, it pushes before it another portion of peritoneum, which becomes reflected after the manner of serous membranes, to form the tunica vaginalis testis.' This, however, is an accidental adjunct, dependent upon the descent of the testis.' The constant and proper covering, 'tunica albuginea,' consists chiefly of the white sclerous tissue: the spermatic vessels ramify therein, especially the veins, so locally as to facilitate the separation of the tunic into an outer dense protective layer, and an inner laxer layer, the seat of the minuter subdivisions of the arteries proceed

ing to, and of the venules returning from, the essential parts of the gland. Processes of the inner layer, resolving into areolar tissue, convey the vessels into the gland-substance, and partition that substance into lobes: a denser layer is continued from the line of the albugineal tunic perforated by the testicular vessels, and projects some way into the gland: it is called corpus Highmori,' or mediastinum testis,' and varies in longitudinal extent, and depth of position, in different Mammals: in Man it is limited

501

to the tract, fig. 501, b, along which the reticulate ducts emerge or become efferent.' The cavities in which the sperm-cells are developed, fig. 514, have the form of tubes, of a diameter of from 0 to 00 of an inch, minutely and extensively convoluted: from two to five of such tubes, averaging two feet in length in the human testis, are packed into a long pyramidal lobule, invested by a process of the inner albugineal tunic: and the sum of these lobules or packets of seminiferous tubules forms the glandular part of the testis, fig. 501, a, a. The reticulate intercommunication manifested in the wider spermogenous tracts of the milt of fish (vol. i. p. 569, fig. 379), prevails in the more finished and thickcoated seminal tubules of Mammals: and, where such become free, they have blind ends. From the lobules the tubules converge, anastomosing, but with straighter course, to the mediastinum, and there form the plexus called rete testis,' ib. b. From this the vasa efferentia,' ib. c, c, emerge, and enter the upper end of the appended body called epididymis,' ib. d, g. Here the convoluted disposition of the tubule is resumed, and from ten to twenty groups, called 'coni vasculosi,' resembling, save in the greater width and less length of the tubuli, the lobes of the testis, combine to form the head or globus major,' d, of the epididymis. By repeated anastomoses there, a single tube results, the trans

Glandular structure of Haman testis, as shown by mercurial injection, nat. size. CCLXXII".

6

6

versely disposed convolutions of which, f, form the rest of the epididymis: at its lower part the tube naturally untwists, and increasing in size, ib. g, i, becomes the spermduct or vas deferens,' k. In Man and many lower Mammals another feature of the spermogenous tract is commonly shown in the epididymis by the offset and blind termination of one or more tubules, as at h, fig. 501.

The epididymis varies in relative size and position to the testis in different Mammals. In all, the semen is conducted, in coitu, by a single intromittent organ traversed by a complete canal, which may bifurcate at its termination in the lowest members of the class. Accessory secretions are added to the semen at the beginning of the urethro-seminal canal, by glands called ' vesicular,' prostatic,' and Cowperian.' But these do not coexist in every species, and the varieties in regard to their presence and development, as well as in the structure and muscles of the intromittent organ, are the chief elements in the comparative anatomy of the Mammalian male generative organs.

§ 369. In Monotremata.These are true 'testiconda:' each testicle, fig. 502, e, e, is situated immediately below, or sacrad of, the kidney, ib. a, and is suspended to that gland by a fold of peritoneum; the same fold is continued to the neck of the bladder, inclosing the vas deferens, fig. 308, a, which appears to be thick and simple, but when injected and dissected, as at f, fig. 502, is seen to be slender and disposed in a series of close transverse folds. In neither Ornithorhynchus nor Echidna

502

is there any disparity of size between the right and left testicle. The vas deferens emerges from the upper and inner part of the testis e; and, from the peculiar extent of its transversely folded disposition, seems to prolong the epididymis nearly to the neck of the bladder; the folds gradually diminish, and the duct itself enlarges, as it approaches its papillary termination, which is in the beginning of the urogenital canal, g. This canal is continued

through the pelvis and terminates in the vestibular passage, anterior to the orifice of the rectum, q. The vascular tissue of the penis commences at the termination of the urogenital canal; it is separated by a median septum into two lateral moieties, and both are inclosed by a common dense fibrous sheath. The whole penis in its collapsed and retracted state is about fifteen lines in length in Echidna, and is concealed in a large preputial sheath. The terminal half of the penis is formed by the glans, which, in Oruithorhynchus, presents a quadrilateral form, l, and is traversed by a median longitudinal furrow upon both the upper and the under surface. Its exterior is beset with numerous short and hard epidermal spines: its extremity is bifurcate, and each lobe is directed outward, and terminates in three or four spines, ib. k, k, much larger, but softer, than the rest, and which are usually retracted in a depression. A longitudinal azygos levator' muscle runs along the upper surface of the penis; it arises by two lateral slips from the internal stratum, ib. n, of the protrusive sphincter, o. Another longitudinal, but longer and more slender muscle, the retractor penis,' ib. p, arises from the base of the coccyx, and is inserted into the origin of the penis near the termination of the urogenital canal. The urethral canal of the penis begins by a small orifice at its root, communicating with the termination of the urogenital passage, and by the combined action of the last described muscle with the sphincter cloacæ' it can be brought into contact with the terminal papillæ of the spermducts. Such temporary continuation of the urethral and seminal passages takes place during the vigorous muscular and vascular actions of the parts in coitu, the semen being then propelled from the one along the other without escaping into the cloaca. Under ordinary circumstances, as when the urine is transmitted along the urogenital passage, that fluid escapes into the vestibule, and may there be blended, as in the Bird, with the rectal excrement. The seminal urethra, commencing by the distinct aperture above described, is about a line in diameter, and continues single to the middle of the glans, where it divides into two canals; each branch runs along the middle of the bifurcation of the glans, and, when arrived at the base of the large papillæ, subdivides into smaller channels corresponding with the number of the smaller ones, and opening upon their apices. If the canal of the penis were slit open along its under part, and thus converted into a groove, the male organs of the Ornithorhynchus would be like those of a Tortoise; and although the Mammalian type of intromittent organ is manifested by the completeness of the urethral canal, a resemblance to that of Lizards is