A Letter to Dr Edward Tyson. Giving an Acconnt of the Anatomy of Those Parts of a Male Opossum That Differ from the Female. By William Cowper, F. R. S.

II. A Letter to Dr Edward Tyson. Giving an Account of the Anatomy of those parts of a Male Opossum that differ from the Female. By William Cowper, F.R.S. Sir, Had your Account of the Female Opossum been less Accurate, I should not have been tempted to look farther than the Parts that distinguish the Sexes; for on comparing your Description of the Organs common to both, it was sufficiently evident how little can be added, tho you had but one Subject to examine. The singular contrivances of that Animals Organs renders the Anatomy of them very desirable, I may say entertaining, to those who have Tastes for such enquiries. Comparative Anatomy (as instructive as it is) does not escape the Censure of the Vulgar; tho you know the greatest Illustrations of the use of Parts are not only to be had from thence, but the very existence of divers Organs in Human Bodies have been made known to us by Discoveries first made in the Bodies of Quadrupeds. The Circulation of the Blood, and the Passages for the Chyle and Lymph, had been as little known to us as our Predecessors, were it not for Dissections made on the Bodies of several Animals. But I shall no more abuse your Patience with these kind of Apologies, than I would omit owning an obligation incumbent on me, (which is) to beg your pardon for making an attempt, where you have given such instances of your great Ability. This This Male Opossum, as the Female you dissected, was brought from Virginia and presented to the Royal Society, by the same Benefactor, William Bird Esq; and was also kept alive in their Repository; but falling from its mea (like that you examin'd, I guess) it languished and dyed: The cause of its Death appeared to be from a Mortification of the Duodenum immediately below the Pylorus, which seemed to arise from a quantity of Hay, that had been collected in the Stomach, and matted together in the shape you have described, and figured the hairy Tophus you found in the Stomach of that you dissected, but I could not find any hair in this; this wad of Hay slipping out of the Stomach stuck in the Duodenum, which together with the viscid matter that involved it, completely obstructed the Passage in that Gut, as well as that of the Gall into the Gut, which appeared from the Distention of the Liver as well as fullness of the Gall Bladder. The Omentum, which in this Creature is only fastened to the bottom of the Stomach, had also suffered a Gangrene, as had almost the whole Canal of the Guts: but of this by the by, my design being only to give you an account (such as it is) of those Parts of the Male, which distinguish it from the Female. Besides the Organs employ'd in Generation, the Male Opossum differs externally from the Female, there being no Marsupium or Pouch to receive the young ones, which you have given so exact a description of; nor are there any Muscles inserted to the Skin of the Abdomen springing from the Osseous Marsupialia, as you call the Bones, which may deserve the Name of Hyoides, from the figure they make with the Osseous Pubis of this Animal; which Bones do not seem to differ in the Male, from those of the Female you have described and figured in the Transactions above-mentioned. There is no external appearance of Genitals in the Fig. 1 Male Opossum but the Scrotum; which, is but just big enough. nough to contain the Testes; nor could I readily discover any other Foramen outwardly in these parts but the Anus, A, which leads to the Rectum; but on withdrawing its sides, I found another Foramen, B, which on Dissection appeared to be the Preputium or Outlet of the Penis. On compressing the parts on each side this Cloaca, A, B, I observed two Drops of yellowish colour'd Liquor (of the resemblance of Pus) start out on each side the Anus, c.c., which on further examination I found come from two glandulous Bodies or Bags placed on the Sphincter Muscle of this Part. This sort of Liquor (it seems) you found in the Pouch of the Female, which, like this, had more of the peculiar Factor of this Animal, than any other part besides; for on removing these Parts with the Skin about the Cloaca, I was freed from the ungrateful Smell of it. On separating the Skin from the Muscles of the Abdomen, the two above-mentioned Bones (peculiar, I believe, to this Animal) appeared, from whence some Muscles sprang, and were inserted to the Ossa Femorum, which performed the Office of the Psoas Muscles in other Animals, which last named Muscles were much smaller in this than in other Creatures. The Abdominal Muscles were also fastened to the last mentioned Bones, particularly the Recti, which enabled this Creature to project or spring its Body, especially in pulling its hind Legs forward, with more advantage or force than other Animals, which are without these Bones. Immediately under the Skin about the Cloaca, I found a thin fleshy Muscle, inclosing the Preputium, and lower parts of the Rectum and Odoriferous Bags, together with the four Mucous Glands, MM N N, at the roof of the Penis, and body of the Penis itself A; all which parts were liable to be comprisè by the Action of this Muscle, especially when the Penis is erected, whereby its Erection is sustained, by compressing the two external Veins on the Dorsum Penis, of which more hereafter, when I come to speak speak of the manner the Penis of this Animal is erected. On removing this thin broad Sphincter Muscle, I was obliged to clear away two Lumps of hard Fat before the Body of the Penis could be discovered; but we shall leave these Parts till we have cleared the Testes. The Scrotum being removed, each Testicle appear'd as represented on the left side Q T V. the Vasa prepanantia Fig. 2. and Deferentia Q Q being inclosed in the Cremaster Muscles P P. These Muscles were proportionably very large in this Animal, as I have always observ'd them in Creatures, that have no Vesiculae Seminales, which is the Case of this Animal, and this Provision of Nature seems not only necessary to suspend the Testes, but these inclosing Cremaster Muscles, also compress the Epididymides and Vasa Deferentia, and oblige them to dispatch their Contents (the Semen) into the Urethra in the time of the Coition, which otherwise would have a slow progress; but this contrivance appears more peculiarly requisite in this Creature, because the defect of the Vesiculae Seminales here seem to be supplied by the largeness of the Epididymides of the Testes W W Fig. 2., which you know are the excretory Ducts of the Testes, and appear in this Animal to have a larger Bore than ordinary: For this reason the Tunicae Vaginales are very freight in this Animal, as appears in the Figure T V R R. On discovering the Originations of the Spermatick Arteries, I was surpriz'd to meet with an appearance I never heard of nor observed before; and in this I should not have had any satisfaction, if I had not first injected Wax into the Trunks of the great Artery i i i, and Vena Cava h below the Diaphragm. It seems the descending Trunk of the great Artery, below the emulgent Arteries in this Creature, is placed directly under the Trunk of the Vena Cava, nor does the Iliack Branches of the Arteries here twine about those of the Veins, as in Human Bodies and some Quadrupeds, which is done perhaps to compress the Channels of the Veins, by means of the Pulsation of these Ar- Arteries to drive up the Blood in the Veins towards the Heart; but that contrivance seems no way necessary in this Animal, because the contrary position of its Body is more customary in hanging by its Tail with its Head downwards: It is not unlikely, if the Veins of this Animal were examined below the Heart (which indeed I did not think of till those Parts were thrown away) but we should meet with some Contrivance to prevent the Precipitate Flux of the Blood in that Pendulous Position, as I have observ'd in the Trunk of the Cava immediately above the Liver in Dogs. But to return to the Spermatick Vessels. The Arteries arise from the forepart of the Descending Trunk of the Great Artery, and pass through a very small Perforation made on purpose in the Vena Cava, and descend straight to the Testes, as in Human Bodies, and are not contorted in their progress, as we find them in most, if not all Quadrupeds. This Perforation of the Cava perhaps was not only made for transmitting the Spermatick Arteries, but may also frame an Anulus, that may check the velocity the Blood would otherwise have in those Arteries, which rapid Motion of the Blood we find Nature studiously avoids in the Testes of all Animals: For in Men we see these Spermatick Arteries (contrary to all other Trunks of Arteries) are less at their Originations from the Great Artery; and in Quadrupeds (except in this) the Spermatick Arteries are contorted before they reach the Testes, as I have elsewhere taken notice. The Spermatick Veins, after leaving the Testes of this Animal (like those of Humane Bodies) have several Divisions and Inosculation, which are all reduced to one Trunk on each side, and empty themselves into the Cava immediately above the Perforation. Had the known Structure of the Testes, in relation to their Excretory ducts been left undiscovered till now, the bare inspection of those parts in this Animal would instruct Instruc: for on dividing the Tunica Vaginalis (RR) Fig. 2, 3. I found the inclosed Testicle and its Epididymis lying loose, insomuch that they parted from each other as express W X Y Z, and with the assistance of a pretty large Convex Glass I could see the Excretory Duct Z arising from one end of the Testicle, where the Spermatick Artery and Vein Y may be seen: After that Duct has marcht a little way it may be seen folded up into the Body call'd Epididymis WW. and at length makes the Vasa Deferens SS. You know in Men, and most, if not all Quadrupeds, the Epididymides and Testicles cleave so to each other, that without some Dexterity in Dissection the rise of them from the Testes is not to be discovered. This proves to Us the Use of Comparative Anatomy in detecting the Structure of parts which is very Obscure in other Subjects as well as in Humane Bodies; but to return to the Vasa Deferentia, SS. after they leave the Præparantia ab, as in Men and other Creatures, they grow somewhat larger, but on crossing the Ureters e e become less again at their Entrance into the Urethra, immediately below the Neck of the Bladder; where their Orifices could be perceived on each side a Caruncle: Nor are there any Vesiculae Seminales near the Vasa Deferentia of this Animal, as in Boars, Bulls, Horses, &c. which nevertheless cannot be allowed to communicate with each other as in Men; for tho the Vasa Deferentia and Vesiculae Seminales of those last named Animals empty themselves into the Urethra at the same Orifices with the Vesiculae Seminales, yet their Communicant Ducts are so very short, that whatever comes by the Vasa Deferentia will sooner escape into the Urethra, than be received by the Vesiculae, as in Men. The length of the Urethra between the Bladder and the Penis exceeded four Inches, more than three Inches and an half of which was inclofed with a Glandulous Body, Analogous to the Prostates in Men and other Creatures; tures; the Orifices of the Secretory Ducts of this Glandulous Body are very numerous, and open into the Urethra on all sides, as appeared on opening the Urethra, and compressing this Glandulous Body or Prostate, I saw its Secreted Juice start out. Fig. 2, 3. This part of the Urethra I K K L thus inclosed with the Prostates, being very much contorted or folded, in its Natural Situation between the Bladder and the Penis, when there is no Erection, must necessarily be drawn out, and becomes straight when the Penis is Extruded, (which I shall shew by and by happens upon an Erection) by which means this Glandulous Body is necessarily comprèst, and the Succus Prostatarum forced into the Urethra. The Prostate of divers Animals are comprèst by Muscles fram'd on purpose that inclose them, as in Boars, Rams, &c. in Men they are comprèst by the Musculi Levatores Ani. At the root of the Penis of the Opossum we meet with four Glandulous Vesiculae M M N N two on each side, which empty themselves into the Urethra, and contain a Mucous matter, like that I find in the Glands I lately discover'd in this part in Men. These Vesiculae are not only comprèst by the thin broad Sphincter Muscle above mentioned, but the Bulbs of the Cavernous Bodies of the Penis CC, and Urethra EE, when distended (in the Erection of the Penis) also compress these mucous Bags. This compression is effected in Men by the Intumescence of the Bulb of the Cavernous Body of the Urethra*. In Boars, Rams, Cats, &c. we find Nature so solicitous to discharge the contents of the Excretory Ducts of these Glands, that (like the Gizard of Birds) each Mucous Gland is inclosed with a proper Muscle to compress it. The Penis fell next under my Examination, the Fabrick of which appears not less surprizing, than that you met with in the Uterus of the Female; and in many circumstances differ'd from what I have found in all the Animals that that I have hitherto dissected: Besides the Forked Glans of its Penis, B B. its Cavernous Bodies D D had no Connection with the Osse Pubis, nor did the Muscles call'd Erectores or Directores C C cleave to any Bone as in Men and Quadrupeds, but all those parts lay loose under the Osse Pubis. The other extremities of the two Corpora Cavernosa Penis are received into the Glans. Nor did the Corpus Cavernosum Urethrae or its Muscles E E cleave to the Sphincter Ani, as in most other Creatures, but the whole body of the Penis lay loose between the bones of the Pubis and the Rectum, so that on the Intumescence or Erection of the Penis, it is at liberty to be extruded from its Preputium, wherein it is secured from outward injuries when not erected. To favour this Extrusion of the Penis in this Animal, the Urethra I K L is not only very long between it and the Bladder O O, but I found it much more contorted or folded in acuter Angles, than is express'd in the Figures, else the Penis could not be extruded, but the Bladder O O must follow it. Besides it appears, Nature design'd this extrusion of the Penis of this Animal in its Erection, because we meet with Instruments to withdraw it again into the Preputium. f f G shews a pair of Muscles elegantly framed for that purpose on the fore part of the Penis; they arise fleshy from the Corpora Cavernosa Penis D D, and becoming tendinous f f, as they pass through two Ligaments or Pulleys on the Osse Pubis, and are afterwards united into one Tendon G, which is inserted to the upper part or Dorsum Penis. Besides this pair of Muscles (which is peculiar perhaps to this Animal) I found another pair of Muscles H H, that also withdraw the Penis arising from the Rectum, and are inserted to the extremities of the Corpora Cavernosa Penis: In Cats, Male Porpes, Bulls, Rams and Boars, we meet with two Ligaments springing from the Os Sacrum or Ilium on each side, and inserted to the Corpora Cavernosa Penis of those Animals, which like these Muscles serve to withdraw the Penis of those Creatures into the Preputium. Fig. 2, 3. The Corpora Cavernosa Penis of the Opossum differ in their figure from what we find in other Creatures; their upper parts are bulbous D D and covered with Muscles C C like the Bulb of the Cavernous Body of the Urethra in Men: In other Animals, those parts of the Corpora Cavernosa Penis are of a Conical figure. The Muscles of the Cavernous Bodies of the Penis of this Creature having no connection with the Os Pubis, cannot apply the Dorsum Penis to the last nam'd bone, and compress the Vein of the Penis, whereby to retard the Refluent Blood, and cause an Erection, as we have observed in other Creatures; but some large Veins of the Penis here, take a different Course and Pass through the middle parts of the Bulb K K C, and are only liable to the Compression made by the Intumescence of these Muscles C C, that inclose them. Fig. 4. But the chief Agent in continuing the Erection of the Penis in this Animal, is the Sphincter Muscle of its Anus, or rather Cloaca, to which the broad Sphincter Muscle above-mentioned is continued, and does somewhat contribute. When the Penis is extruded from the Cloaca (which must happen when it is erected) the Sphincter of that part necessarily embraces it, the like must be done by the Sphincter Muscle of the Cloaca of the Female in Coition: On these accounts I am apt to think, these Creatures are not very quick in that Act. Besides the figure of the Penis, Fig. 4. shews an unfitness for its retraction till there is a Detumescence of its Glans A B, which perhaps does not happen in these Creatures till both Male and Female are satiated, as in Dogs and other Animals that have Bones in their Penis, and have a bulbous Intumescence of the Glans in Coition, and no Vesiculae Seminales as in this Animal, and also impregnate the Female with more than 2 or 3 at a time, as this does. As the Bulb of the Cavernous Body of the Urethra in Man, is fram'd for the use of the Glans, to keep it sufficiently distended when required, so it seems it is necessary to have two of those Bulbs inclosed with their particular Muscles E E in this Animal, to maintain the Turgefs Fig. 2. cence of its doubled or forked Glans A B when the Penis Fig. 4. is erected: In this distention of this Glans Penis of this Creature, the middle part of the Orifice of the Urethra (in which you see the Probe passing out of Fig 3.) is necessarily comprst, as represented Fig. 4. D; and two distinct Apertures C C are left, as appears by the last mentioned Fig. 4. A B on each side its forked Glans. They that fancy an Aura Seminalis of the Male, passes by the way of the Bloud of the Female to their Ovaria to fecundate the Ova, will here meet with an Instance I must leave them to solve. For to what end has Nature been at the trouble of making double Emissaries for the Semen of the Male Opossum, tho she design'd the Impregnation of a double Uterus of the Female? Certainly one passage in the Glans Penis would have been sufficient to convey the Semen Masculinum to the Mass of Bloud of the Female in the manner they conceive. Nature would never have been at the trouble of all this Clutter in this Animal, in making a double Glans, and contriving two distinct Apertures in the Glans, when its Penis is erected, if the Propagation of the Species had not depended on't: Doubtless 'twas for that end chiefly, that the Penis of this Animal differs so much from what we meet with in other Creatures. Nor could the Penis of this Animal in these Circumstances, be expos'd in a Prepuce, as in other Quadrupeds, by reason of the numerous Accidents that would certainly attend it in this Animals way of living: Nor could its Penis been thus withdrawn, when not erected and sufficiently extruded, when it is if (as in other Creatures that are retromingent also) the Penis here had been fastned to the Osse Pubis. Thus, Thus, Sir, we see Nature in these Instances, as you must have frequently taken notice of in others, accomplish the same ends by different Methods. Although there are no Vesiula Seminales in this Animals, as in Dogs, Weasels, &c. yet we find its Penis without a Bone in it, as in those Creatures; but then we meet here with additional Contrivances to maintain its Erection: Not only the Sphincter Muscle of the Cloaca of the Male Opossum, but that of the Female also closely embraces its Penis in Coition, and effectually retard the refluent Blood from its Corpora Cavernosa, by compressing the Veins of the Penis E. Nor could the Penis of this Animals be fram'd like that in Boars, Rans, Bulls, &c. in whom the Corpora Cavernosa are too large, when not erected, to be secured within the Cloaca of this Animal. If in this I have been tedious, it may be some excuse, I had not time to make it shorter. Who am, Tour Obliged Humble Servant, William Cowper. The Explanation of the Figures. Fig. 1. Shews the external appearance of the Genitals of the Male Opossum, somewhat less than the Life. A B c c. The Anus or Cloaca. A its lower part which leads to the Rectum. B its upper part or Orifice of the Preputium, whence the Urine and Penis is extruded. c c. Two small Apertures, whence the yellowish colour'd Liquor, that had the peculiar Fætor of the Animal, had its Exit. D. The D. The Scrotum just large enough to contain the Testes. E. That part of the Abdomen, where the Marsupium is seen in the Female, which here appears a little more deprest than in other Animals, but cannot retain the young ones, as does the Pouch of the Female. F F. The two Thumbs of the hind Feet, or Hands. Fig. 2. The fore parts of the Organs of Generation dissected from the Male Opossum; done as big as the Life. A A. The Body of the Penis. A B. The forked Glans C C. The Muscles Analogous to the Directores Penis in Men and other Creatures, which here inclose the Bulbi of the Cavernous Bodies of the Penis. D D. The two Corpora Cavernosa Penis before they joyn and make the Body of the Penis. E E. Parts of the two Bulbs of the Cavernous Body of the Urethra. G f f. A pair of Muscles, whose two Tendons f f pass through two Ligaments or Pulleys on the Osse Pubis, and are afterwards united into one Tendon G. inserted to the Dorsum Penis, and serve to draw the Penis within the Cloaca after an Erection. H H. Two other Muscles which serve for the same Use, and arise from the Rectum, but are fixt to the opposite part of the Corpora Cavernosa Penis. I. The Urethra where it has no Glandulous Body inclosing it. K K. The Prostrate or Corpus Glandosum, inclosing the Urethra, which lies contorted between the Penis and Bladder of Urin in the Pelvis of the Abdomen of this Animal. M N. Two Mucous bags on each side, at the root of the Penis, which empty themselves into the Urethra. O O. The Bladder of Urine. P P. The Musculi Cremastreris. Q Q. The Q Q. The left Cremaster Muscle inclosing the Tunica Vaginalis. R R. The Tunica Vaginalis of the right side, opened to shew the inclosed Vasa Preparantia and Vas Deferens. S S. The Vas Deferens. T V. The Tunica Vaginalis inclosing the left Testicle, with its Epididymis V. W X Y Z. The right Testicle, as it appear'd on opening the Tunica Vaginalis. W. Its Epididymis. X. The Body of the Testicle. Y. The Spermatick Vein and Artery as they pass to and from the Testicle. Z. The excretory Duct of the Testicle, which could be distinctly seen arising from the Testes and marching to the Epididymis W, where it is folded up and constitutes that Body, whence it is continu'd to the Bladder of Urin, and call'd Vas Deferens S S. a a. The Spermatick Arteries arising from the fore part of the descending Trunk of the Artera Magna, where they have a common Ductus, which is divided as it passes through an Aperture * made on purpose in the Trunk of Vena Cava. b b. The Spermatick Veins at their entrance into the Cava. d d. The Kidneys. e e. The Ureters. g g. The Emulgent Veins. ψ Part of the left Emulgent Artery. h. The Vena Cava below the Liver. i i. The descending Trunk of the Great Artery. k k. The Mesenterick Arteries. l. The lower Mesenterick Artery, which in this Animal does not arise from the Great Trunk. m. The left Glandula Renalis, that of the right side being placed behind the Trunk of the Vena Cava n. o. A A common Trunk of an Artery, from whence springs the Galtrick, the Superior and Inferior Mesenterick, and the Emulgent Arteries of this Animal. The design of Nature in confining all those Arteries to one Trunk in this Animal, might be perhaps in favour of its usual posture in hanging by its Tail, with its head downwards. This Trunk of the Arteries of the Viscera of the lower Belly, having so many united forces, is the least liable to any Compression that might be made by the contain'd parts of the lower Belly in that Posture. Fig. 3. The back side of the Genitals of the Male Opossum. A. The Body of the Penis. B. Its Glans. C C. The Bulbi of the Corpora Cavernosa Penis covered with their Muscles. D D. The Corpora Cavernosa Penis. E E . . . The two distinct Bulbs of the Cavernous Body of the Urethra, inclosed with their particular Muscles. F F G. Parts of the Muscles express'd on the fore part of the Penis in the preceding figure. H H. The other pair of Muscles springing from the Rectum, and inserted to the sides of the Corpora Cavernosa Penis. I K L. The Urethra covered with the Prostate K L K. M N. The two Mucous Bags on each side. O. The Bladder of Urine. P. The Musculus Cremaster. Q. The Tunica Vaginalis open'd. R. Vasa Preparantia cut from the great Trunks. S S. The Vas Deferens on each side. W X Y Z. The left Testicle, as in the preceding figure, with the opposite side here towards you. e e Parts of the Ureters. * * A Probe inserted into part of the Urethra. K k k k k k k k Fig. 4: Fig. 4. The fore part of the Penis, as it appears when its Corpora Cavernosa are fill'd with Mercury and dry'd; figur'd as big as the Life. A B. Its forked Glans. C C . . . The two distinct apertures that appear in this Distention or Erection of its Corpora Cavernosa. D . . . The middle part of the Orifice of the Urethra, which is occluded on the Intumescence or Erection of the Penis. E . . . The two Veins of the Glans, which are comprast by the two Sphincter Muscles of the Male and Female in Coition. F. The Bulbs of one of the Cavernous Bodies of the Penis distended. G. One of the Bulbs of the Cavernous Body of the Urethra also distended. These Bulbi were open'd on the other side, Ψ to fill the Cavernous Bodies with Quick-silver, but are all express as they ought to appear on both sides in the following Figure. H. The Urethra. I. The Muscles dried, express Fig. 2 and 3. F F f f G. K k: The Veins tyed up to keep in the Mercury, as they pass the Muscles of the Bulbi. Fig. 5. The back part of the Penis express in the preceding Fig. A B. Its forked Glans. E E. Parts of the Veins arising from the Glans. F F. The Bulbs of the Cavernous Bodies of the Penis. G G. The two Bulbs the Cavernous Body of the Urethra. H. The Urethra. K K k k. The Veins tyed up, as they pass out of the Bulbi to keep in the Mercury. IV. Tracta.