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=A Contribution to the Anatomy and Development of the Venous System of Didelphis Marsupialis - 1=

By

Charles F. W. Mcclure.

Professor of Comparative Anatomy, Princeton University.

With 5 Colored Plates and 11 Text Figures.

* The writer wishes to express his thanks to Stephen S. Palmer of New York, for his generosity in supplying the funds necessary to cover the cost of the Plates in this article. Also to his assistant C. F. Silvester thanks are due for his valuable assistance in connection with the preparation of the material used in the investigation.
==Introduction==

It has been found necessary to publish this paper in two parts. The first part deals with the anatomy of the venous system of Didelphys marsupialis, while the second part, which appears later, will deal with the development of the veins.


In 1900 the writer<ref>On the basis of priority of nomenclature tlie specific name marsupialis has been substituted for virginiana. All of the opossums made use of were captured in the neighborhood of Princeton, New Jersey.</ref> published a preliminary article entitled "The Variations of the Venous System in Didelphys virginiana," in which was described a set of variations that occurred in connection with the mode of origin of the postcaval vein. These variations were so unusual in character and occurred with such regularity in all of the opossums examined that a further investigation was deemed necessary. This investigation is now completed and after the examination of one hundred and one (101) opossums the writer can reiterate the statement made in the preliminary article, that the mode of origin of the postcava is so variable in Didelphys marsupialis, that it is impossible to assign any one mode of origin for this vessel that may be regarded as typical of the species.


It is astonishing how little has been published upon the anatomy of the venous system of marsupials when one considers the unique character of their postcaval vein. Previous to 1893 the writer has, with Hochstetter, 93, been able to find in the literature but one reference to the postcaval vein (Owen, 66), in which it has been described as occupying a position ventral to the abdominal aorta, a position which is unusual in mammals.


Previous to 1893, so far as known to the writer, only the following have contributed to the anatomy of the venous system of the marsupials: Martin, 36, Owen, 35, 36, 39-47 and 66, Forbes, 81, and Cunningham. 82. From 1893 up to the present time, Hochstetter, 93, Beddard, 95, Parsons, 96, Windle and Parsons, 98, and the writer, 00, 01 and 02, Of all the above-mentioned investigators Hochstetter was the first to give an accurate and comprehensive description of the anatomy of the postcaval vein for a large number of marsupials.

In dealing with the anatomy of the venous system of Dulclphys it will be the purpose of this paper not to enter into a detailed description of all the veins, but rather to give an account of the general arrangement and principal variations of the caval veins and their chief tributaries, in order that the description may serve as a basis for comparison with the veins of other mammals. In addition to this, the main features of the heart will be discussed and a general comparison drawn between the venous system of Didelphys and that of other marsupials.

==The Heart==

The heart of Didelphys presents only a few characters in which it differs from that of other mammals, and will be considered under the following topics : (1) The fetal structures; (2) the auriculoventricidar valves; (3) the pulmonary veins, and (4) the coronary veins.

1. The Fetal Structures. — A fossa ovalis, annulus ovalis and ductus arteriosus are wanting in Didelphys as in all other adult marsupials ' as hitherto described by Owen, 66, Cunningham, 82, Eose, 90, Parsons, 96, and Parsons and Windle, 98. Eose has explained the absence of the fossa and annulus ovalis in the heart of the adult marsupial on the ground that in the embryo the two auricles communicate with each other, as in birds and monotremes, by means of a number of small openings which are secondarily formed, and which close up early in correlation with the abbreviated intrauterine life of these animals.

2. The Right Aurwuloventricular Valve. — The right auriculoventricular valve of Didelphys consists of one medial or septal and two lateral membranous cusps which are continuous at their bases round the auriculoventricular orifice. The left lateral cusp is the largest, the septal next, while the right lateral is (|uite small and is only with difficulty to be distinguished from the left lateral cusp. The two lateral cusps are attached by means of chordae tendinege to thres musculi papillares which spring from the septal wall; the medial or septal cusp is, for the most part, attached directly to the septal wall by chordae tenilinese; a few of the latter may, however, be inserted into the smallest of the musculi papillarcs which springs from the right side of the septum.


* 3 With the possible exception of Peramehs in which an allantoic placenta is present.


The right auriculoventricular valve of marsu|)ials has Ijccn described by a number of investigators, and it appears from their descriptions that a considerable difference exists not only as to the number of membranous cusps, but also as to the number of musculi papillares that may be present. These differences are clearly brought out in the following table, which explains itself.



The Bight Auriculoventricular Valve of Marsupials.



Y a m i I y — Macropodid.e.

Macropus (spec.)

(Owen, 66.) Macropus rufus.

(WiNDLE AND PaRSONS, qS.)

Dendrolagus bennetti.

(Beddard, 95.) Petrogale xanthopvu.

(Parsons, 96.) Petrogale penicillata.

(Beddard, 95 )

Fa m i ly — Phalangerid.^,

Pha.scolarctos fuscus.

(Martin, 36.) PhascoJarctos cmereus.

(Forbes, 81.) Phalarigista rulpina.

(Cunningham, 82.) Phalangista maculata.

(Cunningham, 82.) Phascoloniys wombat.

((3WEN, 36.) Phascolomijn wouibnt.

(RiisE, 90.)



No. of cusps. Not ffiven.



Not given.



Not given.



Not sriven.



Not given.



Not given.



No. of musculi papillares. 3



2 or 3



One large, one mediumsized and several small.



Where the number of cusps is not mentioned by the author, in all probability, the usual number, three, was observed.

In the above table as well as in the following pages of this article the writer, in order to avoid confusion, has followed the nomenclature adopted by the authors referred to.



Venous System of Diddphyn Marsupialis



Fa m i ly — Dasyuiud.e.

Thylacinus cijnocephalus.

(Cunningham, 82.) Thylacinus cijnocephalus.

(Rose, go.) Dasyurus viverrinus.

(Cunningham, 82.) Phascogale calura.

(Cunningham, 82)

Family — Didelphyid^.

Didelphys niarsnpkdis. (McClure.)



No. of Cusps. No. of musculi pupillares.

.5 Two groups.

3 One large, one medium sized and several small 2 3



From the above table, which is supposed to represent the normal conditions, it is seen that the number of cusps that may enter into the formation of the right auriculoventricular valve of marsupials ranges between two and five, and that in one instance {Thylacinus) there appears to be a marked difference of opinion as to the number of cusps present. The question naturally arises: Do those cases in which less or more than three cusps have been observed represent the normal conditions, or do they indicate that the observers have interpreted differently as to what really constitutes a cusp ? It is well known that considerable variation exists among the higher mammals so far as their valve structure is concerned. In some mammals there is scarcely any division into a right and left lateral cusp, so that one would not be far from the mark in describing their valves as consisting of one lateral and one septal cusp. In other mammals the three cusps may be quite distinct and in some instances an extra or supernumerary cusp may be present, being formed, probably, as the result of an extra notching of the free border of one of the lateral cusps. In view of the above statements it appears to the writer that the membranous valves of marsupials must be classed with those of the higher mammals, a classification already adopted by authors of recent text-books (Beddard, 02, and Wiedersheim, 02), although they have given no reasons for so doing.

Before closing this topic it may be stated that the semi-lunar valves of the aorta and pulmonary artery, as well as the mitral or bi-cuspid valves of Didelphys and other marsupials, thus far described, agree in their structure with those of the higher mammals. .

3. The Pulmonary Veins. — There is little to be said concerning the pulmonary veins of Didelphys except that they usually unite to form a V. pulmonalis communis before opening into the left auricle. In a few eases in which a V. pulmonalis communis was not present the veins always opened into the auricle close together.




As a general rule the pulmonary veins of marsupials probably present as wide a range in their method of termination as these veins do in the higher mammals. They may open into the auricle by means of a V. pulmonalis communis as in Didelphys, close together as in Petrogale (Parsons, 96), or in separate groups as in Thylacinus and the wombat (Eose, 90). Owen, 66, as opposed to Eose, states that in the wombat they may open close together or by a single trunk.

4. The Coronary Veins. — The coronary veins of Didelphys consist of a dorsal (posterior) and a ventral (anterior) group. The dorsal group consists of one large and several small veins, which, for the most part, lie upon the surface of the left ventricle and open into the left precava near its point of termination in the right auricle. The ventral group also consists of one large and several small veins. The small veins open directly into the right auricle. The large vein (V. cordis magna, V. c. m. in Text Fig. I ) , which lies in the ventral (anterior) interventricular furrow, does not, as in most mammals, on reaching the auriculoventricular groove deviate to the left and open into the left precava, but pursues a course somewhat similar to that of a coronary vein in birds, by passing dorsad between the root of the pulmonary artery (A. p.) and the left auricle and then deviating to the right over the dorsal surfaces of the roots of the pulmonary artery (A. p.) and aorta (Ao.) to open into the atrium of the right precava (pre. .1.).'

Cunningham was the first, so far as known to the writer, to notice the unusual course of this vein in marsupials (Thylacinus), and regarded its peculiarity as one of the two distinctive characters of the marsupial heart, the other lu-iiig the al)senee of the fossa and aniiuhis ovalis.



Fio. I. Diaai'am of vessels at base of heart. Di(lel}>luix iiKirsii iiidlig. Showing course of V. cordis rnat,^na. Ventral view. Ao., aorta; A. p., pulmonary artery; CS., crescentic notch; pre. d., rig-ht precava; pre. s.. left precava; RA., right auricle; RV., right ventricle; V. c. m., V. cordis magna.



5 In birds [Bnteo boredlis and Soirutteria molUssima) the vein iu question pursues the same course as in JJldelphys with the exception that it does not lie dorsal to the root of the aorta, as the vein opens further to the left into the sinus common to the openings of the postcava and the right precava.



There is one more topic in connection with the structure of the heart that is worthy of mention, since it has been erroneously described by Owen, 66, as a constant marsupial character. I refer to the socalled bifurcated right auricular appendix. It has been described as a prominent feature of the heart by Owen in Macro pus and Phascoloiinjs; by Cunningham, 82, in Phalangista vulpina and P. maculata and by Parsons, 96, in Pelrogale. It has l)een found by Cunningham to be only slightly indicated in Pliascogale and by the writer in Didelphys (C S., Text Fig. I), and to be wanting by Cunningham in Vasyurus and. Thylacinus. The investigations of Cunningham have entirely disproved the claims of Owen as to the constancy of this character for the heart of marsupials, since he found it wanting in Dasyurus and Thylacinus.

A bifurcated right auricular appendix apparently possesses no great significance beyond the circumstance that it represents an instance in which the free margin of the appendix has become notched or invaginated as the result of its proximity to the root of the aorta. Such a notching is not uncommonly met with in the hearts of the higher mammals, as, for example, in Arctomys monax^' in which a well-defined bifurcated right auricular appendix may be .present.

The Veixs of the Head and Neck. {Text Fig. II.)

There are two precaval veins in Didelphys, and this is the rule in all marsupials with the possible exception of Belideus hreviceps, which has been described by Forbes, 81, as possessing only one. In Didelphys each precava (pre.) begins opposite the first rib and is formed through the union of three veins, the V. subclavia (V. s.), the V. Jugularis communis (V. j. c), and a vein which I have designated as the V. costovertebralis (V. cv.). The tributaries of the precaval veins are as follows: (1) A Y. mammaria interna (V. mam.) which opens into the ventral surface of each precava near its union with the subclavian vein; (2) the V. azygos {Y. a.), which opens into the left precava about opposite the head of the third rib, and (3) the posterior group of coronary veins which also open into the left precava, about opposite the head of the fifth rib. The right and left precaval veins open into the right auricle along its antero- and posterodorsal walls, respectively;

"Princeton Morphological Museum, No. 487.


Thc left precaval vein opening in common with the posteava. The internal manunary and the subclavian veins do not present any unusual characters and will not be further considered. The V. costovertebralis will be described in connection with another topic.



Fig. II. Diagram of the veins of the head and neck of Diddphys niarsupiaUx. Tentral view.

a., small veins which collect blood from the submaxillary and sublingual glands; b., a vein which collects blood from the superficial muscles on the front of the neck; c, a vein which collects blood from the side of the larynx; cl., clavicle; do., dorsal portion of venous ring; pre, precava ; X., outer venous ring, formed by external jugular, axillary and cephalic veins ; v., ventral portion of venous ring formed exclusively by external jugular vein; V.a., V. azygos ; V.an., V. anastomotica; V.a.p., V. auricularis posterior; V.ax., V. axillaris; V.cep., V. cephalica; V.c.d., V. cervicalis descendens; V.cv., V. costovertebralis ; V.j.c, V. jugularis communis; V.j.e., V. jugularis externa ; V.j.i., V. jugularis interna; V.mam., V. mammaria interna; V.m.e., V. maxillaris externa ; V.m.i., V. maxillaris interna; V.s., V. subclavia ; V.s.max., V. subraaxillaris ; V.s.ment., V. submentalis ; V.t.s., V. transversa scapulae.



The V. jugularis communis (V. J. c.) is a short trunk and is formed, on each side, through the union of the V. jugularis externa (V. j. e.) and the V. jugularis interna (V. j. i.). The common jugular vein receives one tributary, the V. cervicalis descendens (V. c. d.), which follows the course of the ascending cervical artery and opens into the dorsal surface of the common jugular near its union with the subclavian vein.

The internal jugular vein (V. j. i.) presents no unusual conditions except that it is an exceedingly small vessel as compared with the size of the external jugular (V, j. e.)/

The external jugular vein (V. j. e.) begins near the angle of the lower jaw and is formed, on each side, through the union of three veins: The V. maxillaris externa (V. m. c), the V. maxillaris interna (V. m. i.) and a vein which may be designated as the V. submaxillaris (Y. s. max.). The external and internal maxillary veins often unite to form a V. maxillaris communis before joining the submaxillary vein.

The external jugular vein (V. j. e.), along most of its course, lies quite superficially and instead of passing into the thoracic cavity on the dorsal side of the clavicle, as is usually the case in mammals, it forms about it, on each side, a complete venous ring so that one part of the external jugular lies ventral (v.) and another dorsal (do.) to the clavicle (cl.).* Hochstetter, 96, has described the presence of a somewhat similar venous ring in Ornithorliynchtis and states that the portion of the ring which lies dorsal to the clavicle is much larger than the ventral and forms the main trunk of the external jugular. In Didelphys there is not much difference in size between the dorsal and ventral portions of the ring, as, in the majority of cases observed, they were found to be subequal in size.

The formation of such a venous ring about the clavicle, so far as the writer knows, has not been hitherto described for Didelphys nor for any marsupial. In a specimen of Petrogale recently examined by the w^riter no indication of such an annulus was present, so that in all probability this feature is not of common occurrence among the marsupials.

Of the veins opening into each external jugular the following are the most important: Beginning craniad, (1) one or two veins which return blood from the postauricular region (V. a. p.) ; (2) the V. transversa

' This is also the case in Petrogale (Beddard, 95, and the writer).

■^ As a matter of fact two venous rings are formed about the clavicle in Didelphys ; the one mentioned above, which is formed exclusively by the external jugular vein and another (X.), much larger, which is formed through a contluence of the cephalic vein (V.cep.) with the axillary (V.ax.) and external jugular (V.j.e.) veins. The last mentioned or outer ring (X.) is not uncommon in mammals. It is a prominent feature in the three-toed sloth (Bradnpus tridactylns) and is sometimes met with in man (Nuhnl.scapula (Y. t. s.) ; (3) the V. cephalica (Y. cep.), which arises on the radial side of the hand and forearm (this vein also opens into the axillar}' vein); (4) two veins which may open separately or by a common trunk into the dorsal portion (do.) of the venous ring, one of which (b.) returns blood from the superficial muscles on the ventral surface of Ihe neck, while the other (c), besides uniting with the internal maxillary vein, collects blood from the side of the larynx.

In addition to the above mentioned tributaries, the external jugular veins anastomose with eacb other across the middle line of the neck. This anastomosis may be accomplished in one of two ways: Either by means of two vessels which run between the dorsal and ventral portions of the venous rings of opposite sides and fuse in the middle line of the neck (Y. an.), or by means of a single vessel whicb extends between the ventral portions of the two venous rings as in Text Fig. lY (Y. an.).

The affluent veins of each external jugular are, as mentioned above, tlie Y. submaxillaris (Y. s. max.), the Y. maxillaris externa (Y. m. e.) and the Y. maxillaris interna (Y. m. i.). The submaxillary veins are quite large and anastomose with each other on the dorsal surfaces of the geniohyoid muscles. Each submaxillary vein receives the following tributaries: Yeins from the tongue and the floor of the mouth; the Y. submentalis (Y. s. ment.) and small veins from the submaxillary and sublingual glands (a.).

The external maxillary vein collects blood from the face and the internal maxillary from the regions supplied by the internal maxillary artery.

The Yeixs of the Yektebral Canal and the Deep Lying Yeins of THE Cekvical and Thoracic Eegions." {Text Fig. III.)

Text Fig. Ill is a diagram of the veins of the vertebral canal and the deep-lying system of veins of the cervical and thoracic (first five intercostal spaces) regions. These two regions need a special description, since the relations which exist here among the veins are quite unusual.

The Y\. columnae vertebrales (Yv. c. ve.) consist of two large venous sinuses which lie ventral to the spinal cord and extend, on each side, within and along the entire length of the vertebral or spinal canal. In the region of the first thoracic vertebra they anastomose at one point dorsal to the spinal cord.

"This deep-lying system of veins was largely studied by means of corrosions.




The V. eostovertebralis (V. cv.), previously mentioned as one of the tributaries of the precava, is most intimately connected with this deep-lying system of veins and may, therefore, be considered at this point.

The tributaries of the V. eostovertebralis {\. cv.) are, on each side, as follows : ( 1 ) A small superficial Y. intercostalis suprema (V. i. s. I) ; (2) two large veins, one of which lies just cranial (d.) and the other caudal (e.)









to the first rib (R. I) ; (3) the V. vertebralis (V. v.), and (-f) a deeplying V. intercostalis suprema (V. i. s. 11).

1. The superficial V. intercostalis suprema (V. i. s. I) commonly opens into the V. eostovertebralis, but may, instead, open into the precava. It lies superficially, collects blood from the first intercostal space and usually anastomoses directly with the second intercostal vein (g., right side).

2. The largest tributaries of the Y. eostovertebralis consist of two large veins, one of which lies just cranial (d.) and the other caudal (e.) to the first rib. Both of these veins connect directly with the Yv. columnaj vertebrales (Yv. c. ve.). The vein cranial to the rib is sometimes the larger of the two, though, as a rule, they are subequal in size. These two vessels

superficial V. intercostalis suprema; 

V.i.s.11., deep V. intercostalis suprema; OCCUpy a pOSltlOn m the CCrviCai and

V.j.e.,V.jugularis externa ; V.j.i.,V.jug-u- . . t ,i • ,i t p

larisintorna; V.v.,V. vertebralis; Vv.c.ve., thoraClC rCglOUS directly m the imC Ot Vv. columnne vertebrales.

the segmental vessels and appear to correspond to the last cervical and first intercostal veins, respectively. 3. The Y. vertebralis (Y. v.) lies in the vertebraterial canal with the vertebral artery and opens into the large tributary of the Y. eostovertebralis which lies just cranial to the first rib (d.). It anastomoses, by means of segmentally arranged vessels, on its medial side, with the




Fig. III. Diagram of the veins of the vertebral canal and the deep-lying- veins of the cervical and thoi-acic regions ot DidelphyK marmijialix. Ventral view.

d. and e. tributaries of the V. eostovertebralis which lie cranial and caudal, respectively, to the first rib ; cl., clavicle ; f ., anastomosis between the azygos and intercostal veins and the veins of the vertebral canai; g., vein of the second intercostal space: h., tributary of the deep superior intercostal vein ; pre, precava; K. I-R.VI., the first six ribs ; V.a., intercostal branches of azygos vein ; V.c.d., V. cervicalisdescendens; V.cv.,y. eostovertebralis; V.i.s.I., intercostalis


V\. columnas vertebrales (Vv. c. ve.), and, on its lateral side, with the descending cervical vein (V. c. d.).

^1. The deep V. intercostalis suprema (V. i. s. II) is a comparatively short vessel which runs, on each side, parallel to the vertebral column and extends between the second and sixth to seventh ribs dorsal to ilicir necls. This vein opens into the vein, previously mentioned, that lies caudal to the first rib (e.) and collects blood from the second to sixth intercostal spaces. It anastomoses in this region with the intercostal branches of the azygos (f.) and, opposite the head of each rib, by means of a large anastomosis with the Vv. columnse vertebrales. It also receives two important tributaries: (1) The vein of the second intercostal space (g.), which usually opens into it without previously anastomosing with the azygos branches, and (2) a vein (h.) which collects blood from the deep muscles on the back of the neck and joins the deep superior intercostal vein (V. i. s. II) about opposite the third intercostal space."

In comparing the veins of Didelpliys, as represented by the V. costovertebralis and its tributaries, with the vertebral system of veins in the sauropsida, especially birds, one cannot help being impressed by the similarity that exists between the two. In Didelphys the vertebral and deep superior intercostal veins occupy the same relative positions with respect to the vertebral column and connect with the Jugular and subclavian veins to form the precava, as do the anterior and posterior vertebral veins in birds. In birds, there being no azygos vein, all of the intercostal veins open directly into a posterior vertebral vein. In Didelpliys, also, some of the veins of the cranial intercostal spaces may open directly into the deep superior intercostal without previously anastomosing with any of the azygos branches (see under Azygos Veins).

The Vena Azygos and its Tributaries." {See Text Figs. Ill and IV.)

There is, as a rule, but one azygos vein in Didelpliys and this is situated on the left side. The azygos vein increases slightly in size from behind forward. Its caudal end invariably joins the postcava about opposite the second lumbar vertebra, while its cranial end, after curving round the left side of the aorta, joins the left precava about opposite the head of the third rib. Between its point of union with the postcava and the middle of the tenth thoracic vertebra the azygos lies dorsal to the aorta and, in consequence of this position, the left intercostal arteries in this region pass dorsad on the left side of the vein. Between the tenth' thoracic vertebra and its point of union with the left precava the azygos vein occupies a position dorsolateral to the aorta, and the left intercostal arteries pass dorsad on the right side of the vein.



'"In Bidelphj/s a branch of an intercostal artery accompanies this vein. Cunningham (82) has described the course of this artery in Th>jla<:inus and Dasyuriin bnt did not mention the vein.

11 The azygos system was dissected in twenty-six opossums.



The manner in which the caudal end of the azygos vein joins the postcava was found to be quite variable. It may join the postcava to the left, which is the usual method, or to the right of the aorta; or it may bifurcate into two branches on the ventral circumference of the aorta, one of Avhich joins the postcava to the right and the other to the left of the aorta. In the latter case the

iorta is encircled by a venous
ring formed by the azygos and postcaval veins.

On account of the numerous variations which occur in the cranial part of the thoracic region it is difficult to establish what might be called a typical arrangement of the tributaries of the azygos vein. The arrangement, however, which comes nearest to being typical is represented by Text Fig. IV. Here all of the intercostal spaces on both sides, except the first two, are drained by the azygos system; also the blood from the lumbar region, on the left side, as far eaudad as the third, and, on the right side, as far caudad as the second lumbar vertebra, is collected by the tributaries of this vein. The blood from the rest of the lumbar region is collected by the lumbar veins which open into the postcava. The blood from the first intercostal space is collected, on both sides, by the superficial superior intercostal vein (V. i. s. I, Text Fig. Ill), which opens into the V. costovertebralis ; that from the second intercostal space by a vein which opens directly into the deep superior intercostal (g. and V. i. s. II, in Text Fig. Ill) without previously anastomosing with any of the azygos branches.


Fig. IV. A diagram representing the usual arrangement of the azygos branches in Didelphys marsupialis. Ventral view.

cL, clavicle ; d. and e., tributaries of the \ . costovertebralis; do., dorsal portion of venous ring about clavicle; g., second intercostal vein; pc, postcava; pre, precava; K., rib; S., intercostal space; v., ventral portion of venous ring about clavicle; V. a.. V. azygos; V.an., V^. anastomotica : V.cv., V. costovertebralis ; V.i., V. Intercostalis ; V..i.e.,V. jugularis externa ; V.I.. V. lumbalis ; V. r., V.'renalis; V.s., V. subclavia; V.v., V. vertehralis.



The intercostal tributaries of the azygos anastomose with the Vv. columnse vertebrales in the caudal part of the thorax, and, in the cranial part, with the deep superior intercostal (V. i. s. II), which, as mentioned above, is one of the tributaries of the V. costovertebralis (Text Fig. Ill, f.).

The relation of the azygos tributaries to the first four intercostal spaces was found to be quite variable and this was especially the case on the right side. In five opossums (three males and two females) the most cranial tributary of the azygos, on the right side, collected blood from the fourth instead of from the third intercostal space, as is usually the case, and the veins of the second and third intercostal spaces opened directly into the deep superior intercostal vein without anastomosing with the azygos. The blood from the first intercostal space was collected by the superficial superior intercostal vein. On the left side the arrangement of the veins was similar to that represented in Text Fig. IV.

Two instances were met with in which tributaries of the azygos collected blood from all of the intercostal spaces, on both sides, with the exception of the first and anastomosed with the tributaries of the Yv. costovertebrales which lie caudal to the first ribs (e.).

In thirty per cent of the opossums examined a small right azygos vein was present which opened into the right precava about opposite the head of the second rib. In every instance the vein was a small and insignificant vessel and its tributaries were confined to the first five intercostal spaces of the right side. In the eight animals in which it was found it collected blood in three, from the second intercostal space; in one, from the second and third intercostal spaces; in one from the third intercostal space; in two, from the third and fourth intercostal spaces, and finally, in one, from the third, fourth and fifth intercostal spaces. When the right azygos received tributaries from the third, fourth or fifth intercostal spaces, the tributaries of the left azygos which usually collect blood from these spaces were wanting.

The general character of the azygos veins of marsupials other than DidelpJiys has been described by a number of investigators as indicated below.

Marsupials in wliicli Two Azygos Veins are Present ivliich Open into the Right and Left Precaval Veins, Respectively.

Family — Maceopodidje.

Macropus (spccJ) (Owen, 65); Hahnatnrns bemietti (left azj'gos longer, Beddard, 95): Petrogale (specj) (right azj'gos longer, McClure).

F a m i 1 y — Phalangerid.1^.

Left azygos longer in Fhascolarctos cinereus, Phascolomys wombat, Cuscus, Phalangista and Belideus'^^ (Forbes, 81).

Marsupials in ivhich a single azygos vein is present on the right side. F a m i 1 y — Macropodid^.

Petrogale xanthopus (Parsons, 96); Petrogale penicillata, Macropns rnfus and Dendrolagun hennetti (Beddard, 95).

Family — DASYURiDiE,

Th)jlacinu& njHocephalun (Cunningham, 82).

Marsupials in which a single azygos vein is present on the left side. F a m i 1 y — Piialangekid.e.

Phahmgista maeidata and Ph((htnghta vidpina (Cunningham, 82); Phalangista vulpina (Cunningham, 82) and Fhascolarctos fuscits (Martin, 36).

F a m i 1 y — Dasyurid^.

Dasyuritsviverrinus (Cunningham, 82, and Beddard, 95); Phascogale caliira (Cunningham, 82).

F a m i 1 y — Didelphy id^.

Dklelphys caticricora and azarw (Beddard, 95) and Dldelphgs {virginiana) marsupialis (McClure).

It is evident from the foregoing table that, with the exception of Thylacinus and Halmaturus, a right az3'gos vein prevails in the Macropodida? and a left in the Phalangeridse, Dasyuridge and DideljDhyidffi.

Beddard, 95, states that he found considerable variation among several individuals of Halmatnrus, and the writer is inclined to believe that a single azygos vein is the rule in marsupials and that when two are present the case may be regarded as a variation.

i^It is difficult to reconcile Forbes' two statements that in Belidei(s there is only one precava and that the azygos veins in the same animal open into the right and left precaval veins, respectively.


A connection between the azygos vein and the postcava is apparently not of constant occurrence in the marsupials. Beddard, 95, found such a connection invariably occurring in Didclphys azarce. In Phalangista, however, he found it occurring in only one of several individuals examined and, in this case, on account of its large size, it practically took the place of the postcava. In Dichlphijs the writer has never observed any diminution in size of the postcava on account of its connection with the azygos vein.

There is very little in the literature that refers to the manner in w^hich the blood is collected from the intercostal spaces in marsupials.

Forbes, 81, states that when two azygos veins are present the smaller vein collects blood from the first few intercostal spaces {Cuscus, Belideus, Phascolomys and Phalangista). According to Cunningham, 82, the azj'gos tributaries collect blood in Tliylacinus from the intercostal spaces of both sides with the exception of the first three; the intercostal veins of these three spaces join, on each side, to form a single vessel which opens into the prccava. In Cuscus, Phalangista vulpina, Phascogale and Dasyurus the single left azygos vein collects blood from all of the intercostal spaces on the left side and from all on the right side except the first three; these, as in Tliylacinus, return their blood through a single vessel that opens into right precava (Cunningham). In Petrogale xanthopus the azygos vein receives all of the intercostal veins from both sides of the thorax (Parsons, 96).

In a specimen of Petrogale recently dissected by the writer, although the vessels in this region were poorly injected, the blood from all of the intercostal spaces on the right side appeared to be collected by tributaries of the right azygos. On the left side the intercostal veins from all the intercostal spaces except the first two or three also opened into the right azygos; the veins from the first two or three spaces joined, as on the left side in Tliylacinus, to form a single vessel which opened into the left precava. The small left azygos vein, which was present in addition to the left superior intercostal, was not injected, so that its connections could not be traced. A V. costovertebral is which has been described above as a constant tributary of the precava in Didelpliys was wanting on both sides in Petrogale.

It is evident from what has been stated above that there is little agreement between Didelpliys and other marsupials so far as the veins of the first three or four intercostal spaces are concerned. Some of these differences can imdoubtedly be explained on the ground that they are individual variations. The most interesting and fundamental difference, however, is the circumstance that in Didelpliys the most cranial of the iiilereostal spaces are, in part, drained by a deep-lying vein similar in its relations to the posterior vertebral vein of the sanropsida instead of by tributaries of the azygos, or by a a more superficially situated superior intercostal vein, as is commonly the case in marsupials.



The Postcaval Vein and its Tributaries.

For convenience of description the postcaval vein of Didelphys will be described under the following divisions: (1) The prehepatic division; (2) the hepatic division; (3) the rental division; (4) the postrenal division.

The first three divisions do not present any unusual characters and can be briefly considered ; the fourth or postrenal division, however, is most variable in character and will be dealt with at length. 1. The prehepatic division of the postcava is that portion of the vein which extends between the right auricle and the most cranial of the hepatic veins (PH., Text Fig. V).

The only tributaries which this division receives are the Vv. phrenicse, which collect blood from the diaphragm and open into the postcava in its passage through the diaphragm. These veins are not shown in the photograph.

Owen, 66, has stated that when the posterior extremities are smaller or not larger than the anterior ones, as in the ursine dasyure and wombat, the postcava is somewhat less than the left precava and they appear to terminate by separate apertures in the auricle; but in the kangaroo the proportions of the two veins are reversed, and the postcava more obviously receives the left precava before it terminates. In comparing the conditions in Petrogale with those in Didelphys the writer is unal)le to draw any such distinctions as those indicated by




Fig. V. Photot^Tuiili of a corrosion of the cranial end of the i)ostcava and hepatic and renal veinsof DiiliJitliiis mavKupialis. Ventral view. Princeton Morphological Museum, No. 153.

H., hepatic division of the postcava; K., veins of right kidney; PH., prehepatic division of the postcava; PR., postrenal division of the postcava (only a very small portion of this division is shown in the photograph); R., renal division of postcava.


Owen, aurl lincls that in both animals the left precava opens into the auricle in common with the postcava.

2. The hepatic division of the postcava (H., Text Fig. V), which lies within the liver, includes that portion of the vein into which hepatic veins open. The hepatic veins open into the postcava by means of three large and two or three small branches. The small branches return the blood from the caudate lobe of the liver.

3. The renal division of the postcava (R., Text Fig. V), includes that section of the vein which lies between the most caudal of the hepatic veins and a point just behind the most caudal of the two renal veins, so as to include that portion of the postcava into wdiich the renal veins open.

The right suprarenal body is firmly attached to the dorsal surface of the postcava in this region, and its cranial end is embedded in the caudate lobe of the liver. The only direct tributaries which the renal division receives are the V. suprarenalis dextra, which opens into the dorsal surface of the postcava, and the Vv. renales.

The right renal vein usually opens into the postcava cranial to the left and about opposite the first lumbar vertebra. The left renal vein, which lies just caudal to the Truncus cceliacomesentericus, opens into the postcava about opposite the second lumhar vertebra. The left suprarenal body lies upon the left renal vein into which its vein opens.

Multiple renal veins, especially on the right side, were frequently met with and, in a few cases, the right renal artery was found to cross the postcava on its ventral instead of its dorsal surface.

The first three divisions of the postcava, with the exception of that portion into wdiich the renal veins open, occupy a position, as in other mammals, on the right side of the body ventrolateral to the aorta and at no point come in contact with the latter. At the level of the renal veins, however, the postcava bends mediad so as to reach the ventral surface of the aorta and occupies this position with respect to the aorta as far caudad as the Vv. iliacas extemas.

4. The postrenal division of the postcava (PR.) consists of that portion of the vein which lies caudal to the renal veins (V. r.). (See Text Figs. V and IX and Fig. 23, Plate V.) Its tributaries consist of two or three pairs of Vv. lumbales, the Vv. spermaticae internas and the Vv. iliacse. The caudal end of the azygos vein also joins this division of the postcava. Each pair of lumbar veins may open into the postcava either as single vessels or by means of a common trunk. The internal spermatic veins (V. sp. i.) open into the postcava slightly caudad of a point midway between the renal and external iliac veins, the vein of the right side being slio-htlv cranial to the loft.




In all marsupials hitherto examined, Avith the possible exception of Petaurus taguanoides (Hochstetter, 93), the postrenal division of the postcava lies ventral to the aorta and, in this respect, forms an exception to the conditions met with in other mammals in which a single postcava is normally present. This unique position assumed by the postcava in marsupials was apparently first observed by Owen in 1866 in the wombat and kangaroo, although it is due to Hochstetter's more recent investigations that it may be regarded as characteristic of the marsupials in general.

In its position, ventral to the aorta, the postrenal division of the postcava of Didelphys marsupialis agrees with that of other marsupials. It differs from these, however, in one particular feature, namely, in the variable manner in which its posterior tributaries (Vv. iliacs) unite to form the postcava.

In order to understand these remarkable variations in Didelphys it will be necessary first to consider the anatomy of the postrenal division of the postcava in other marsupials where, so far as known to the writer, its mode of origin is uniform.

In a wombat (Phascolomys mitchelU) and a wallaby [Petrogale (spec.?)] recently dissected by the writer the postcava was formed, in each animal, through a union of the Vv. iliacse communes (V. i. c. in Text Fig. VI), which took place ventral to the arteries. Also, in each animal, between the Truncus hypogastricosacralis (T. hs.) and the A. renalis (A. r.) the postcava was situated in the middle line and ventral to the aorta (Ao.). In Petrogale the Vv, spermaticse internae (V. sp, i,) opened into the postcava, as in Didelphys, slightly caudad of a point midM'ay between the Vv. renales (V. r.) and the Vv. iliacse communes (V. i. c). In the wombat, however, the spermatic veins opened into the postcava slightly caudad of the renal veins.'" Whether this connection, so far craniad, is the usual method in the wombat can only be determined through further investigation. At any rate in Didelphys and other marsupials thus far examined, including the wombat (Hochstetter, 93), the internal spermatic veins open into thepostcava at about the same level as that figured for Pctrogale.


'^In the wombat the spermatic arteries also arose in the neighborhood of the kidneys.




Fig. VI. Postrenal division of the postcava of Petroyale (spec.?). Ventral view.

A. i.e., A. iliaca externa; A.i.i.. A. iliaca interna; A.m., A. mesenterica; Ao., aorta; A.r., A. renalis; A.s.i.a., A. spermatica interna anterior; A.s.i. p., A. spermatica interna posterior ; pc, postcava ; T.hs., Truncus hyp' gasrricosacralis ; V.i.e., V. iliaca externa; V.i.i., V. iliaca interna; V.r., V. renalis; V.sp.i., V. spermatica interna.




In the wombat the renal and spermatic veins anastomose with each other, on both sides, by means of a vessel which follows the ureter." In Didelphys a similar anastomosis was met mth, and in well injected animals the anastomotic vessel could be distinctly traced caudad of the spermatic veins as far as the neck of the bladder. Hochstetter, 93, with the exception of its extension to the bladder, was the first to observe the presence of this anastomosis in marsupials. He has also figured the same as occurring in an edentate, Dasypus novemclnctvs (93, Taf. 23, Fig. 25).

The above description of the postcaval vein in Petrogale and Phascolomys, with the possible exception of the spermatic veins, agrees in all essential details with the findings of Hochstetter and others who have examined this vein in a number of marsupials.

Following is a list of marsupials, known to the writer, in which the postrenal division of the postcava has been described, and in which it has l)een found to be similar in all respects to that of Petrogale :

Family- — •Maceopodid.e.

Hahnaturus gUjantcxs (Owen and Hochstetter) ; Halmaim'ushennetti and Hypsiprymnus (spec?) (Hochstetter); Macropus rufus (Windle and Parsons) and Petroyole j^enh-lllata (Windee and Parsons). •

F a m i 1 y— Phalangeeid.i:.

Phnscolomys wombat (Owen and Hochstetter) ; Phahtngista vulpina, Belideus ariel Sind Ctiscns (Hochstetter).

Family — Dasturid^.

Phascooale jJemciUata (Hochstetter).

Family — Didelphyid^.

DideJphyx laniyera and Didelphys pHxiJJa (Hochstetter).

From what has been stated abo-^o it is evident that the postcaval vein is formed in a variety of marsupials in a definite and uniform manner and that its variations when occurring represent, as in other mammals, exceptions to the general rule. In Didelphys marsupialis, however, the case is quite diJI'ereut. Here instead of occurring as exceptions the variations appear to be tlie rule, so that it is actually impossible in this animal to assign any one mode of origin for the postcava that may be regarded as characteristic of the species. This oj)inion was first advanced by the writer in a preliminary article in 1900, but on the basis of less extensive observations.


i This anastomosis is undoubtedly present in Petrofjale; the vessels, however, were not injected in the animal examined by the writer.


For descriptive purposes the various modes of origin of the postcava in Didelphys marsupialis have been classified by the Avriter under three Types as follows:

Type I. — Those cases in which the Vv. iliacse internae unite with the Yv. iliacae externse to form the postcava,'" ventral to the Aa. iliacae communes or ventral to the aorta.

Type II. — Those cases in which the Vv. iliacae interna} unite with the Vv. iliacae externae to form the postcava, dorsal to the Aa. iliacae communes or dorsal to the aorta.

Type III. — Those cases in which the Vv. iliacse interna? unite with the Vv. iliacae externae to form the postcava, both dorsal and ventral to the Aa. iliacae communes or both dorsal and ventral to the aorta.

So many variations of this last Type were met with that a further subdivision of Type III was found necessary, as follows :

Type III, A. — Includes those cases in which the principal union between the Vv. iliacae internae and externa takes place ventral to the arteries in question.

Type III, B. — Includes those cases in which the principal union between the A^v. iliacae internae and externae takes place dorsal to the arteries in question.

Type III, C. — Includes those cases in which the above mentioned dorsal and ventral unions are about suhcqually developed.

One hundred and one (101) opossums were examined and, in all but two, the different variations presented by their postcaval veins could be classed under the above mentioned three Types. In these two individuals, however, the postcava neither in its position, with respect to the aorta, nor in its mode of formation, conformed to the marsupial type but rather to the type of postcava which is characteristic of the higher mammals.

The following table shows the distribution of the above mentioned Types among ninety-nine individuals (34 males and 65 females) :

i When the postcava is bifurcated, in this as well as in the following- Types, it is the common iliac veins that are formed instead of the postcava.



With the exception of eleven individuals which were about half-grown, the above observations were made upon adults. The variations observed in the half-grown opossums were in all respects similar to those of the adult, and I may state here that the Type of postcava which will be found in the adult is already indicated in the embiyo at the time of its birth. Of the three main Types of variations the third easily predominates, while there is not much difference between the first and second. The table, likewise, does not show any marked distribution of the Types among either sex, so I think it may be stated with certainty that no relation exists between sex or age and any particular Type of postcava.

From a study of their development the writer is now able to account for these variations by showing that they are modifications of a '• ground |)lan " arrangement which is common to the veins in the embryo. This question has already been dealt with in a preliminary paper, 02, and will be more fully treated in a subsequent paper on the development of the veins.



The Variations in the Mode of Origin or the Postcava in Didelphys Marsupialis.

Type I. — Includes those cases in which the Vv. iliacse internge unite with the Vv. iliacEe externse to form the postcava, ventral to the Aa. iliacae communes or ventral to the aorta.

Twenty-nine examples of this Type were met with distributed among eleven males and eighteen females.

See Figs. 1, 2, 3 and 4 (ventral views) and Fig. 5 (dorsal view), Plate T.

The representatives of Type I more closely approach the conditions found in Petrogale and other marsupials than those of any of the other Types. In each case the affluent veins of the postcava unite to form the latter on the ventral surfaces of the arteries, but the manner in which the union takes place is not the same. In Petrogale, etc., two Vv. iliac^e •communes unite to form the postcava (Text. Fig. VI), while in Didelphys it is usually formed through the union of a V. iliaca interna communis with one (Fig. 4) or both (Figs. 2 and 3) of the Vv. iliacge externae.

It is worthy of notice in this connection that in Petrogale, etc., a Truncus hypogastricosacralis ^^ and two Vv. iliacaj communes are the rule, while in Type I the reverse conditions usually prevail, namely, a V. hypogastricosacralis and two Aa. iliacas communes.

One of the most interesting as well as remarkable features of this and all of the other Types is the number of variations that occur within the Type. Figs. 1, 2, 3 and 4 represent the most common of the variations ihat were met with under Type I, but by no means include them all.^'

Type II. — Includes those cases in which the Vv. iliacse internge unite with the Vv. iliacse externae to form the postcava, dorsal io the Aa. iliacse communes or dorsal to the aorta.

Twenty-seven examples of this Type were met with distributed among nine males and eighteen females.

See Figs. 6, 7, 8, 9 and 10 (ventral views), Plate II.'

Tiuo principal sets of variations were met with ivithin this Type:

First, in which the internal iliac veins join both the right and left external iliac veins to form common iliac veins. In this set of variations the internal iliac veins may (Figs. 6 and 7) or may not (Fig. 8) anastomose with each other ventral to the A. sacralis media before joining ihe external iliac veins. The internal iliac veins may also, at the point where they join the external iliac veins, be subequal (Figs. 6 and 8) or very unequal (Fig. 7) in calibre. In the latter case (Fig. 7) the blood from the internal iliac veins is returned to the postcava chiefly through ihe right common iliac vein.

Second, as in Figs. 9 and 10, in which the internal iliac veins anastomose Avith each otlier ventral to the A. sacralis media and then open by means of a single vessel (V. iliaca interna communis) into either the left {Fig. 9) or into the right (Fig. 10) external iliac vein. In the adult, an extraordinary similarity exists between the postcava in Echidna aculeata and Type II in Didelphys. Compare Hochstetter's Fig. 16 (96, Taf. 28) of this vein in Echidna with my Fig. 6 of Didelphys.



i^In the wombat dissected by the writer, two common iliac arteries instead of a Truncus hypogastricosacralis were present.

I'ln a few individuals examined (Fig. 3) the spermatic artery (A.s.i.p.) arose by a single trunk from the right side of the aorta ard soon divided into three branches. Two of these ran to the ovaries in the usual manner. The third, however, extended caudad on the ventral surface of the postcava and then divided, on each side, into two branches which ran respectively to the psoas muscle (ps.) and the bladder (bl.) The last mentioned branch followed the ureter.




Type III. — This Type is a combintion of Types I and II, since it includes those cases in which the Vv. iliacae internse unite with the Tv. iliacae externas to form the postcava, both dorsal and ventral to the Aa. iliacfe commiines or 1)oth dorsal and ventral to the aorta.

Including the three subdivisions, A, B and C, forty-three examples of this Type were met with distrilmted among fourteen males and twentynine females.

As shown by the above table, the representatives of this Type constitute about 43 per cent of the variations observed, which is almost twice that of either of the other Types. Considering the composite character of Type III, however, it is evident that this predominance possesses no particular significance beyond the fact that the veins that lie dorsal and ventral to the umbilical arteries in the embryo, both possess a marked tendency to persist in the adult.

Type III, A. — Includes those cases in wliich the principal union between the \\. iliacae internse and externge takes place ventral to the arteries in question.

Eight examples of Type III, A, were met with distributed among three males and five females.

See Figs. 11, 12 and 13 (dorsal views) and Fig. 11: (ventral view of Fig. 13), Plate III.

Two sets of variations within this Type were met with:

First, as in Figs. 11 and 12 (dorsal), in which the internal iliac veins anastomose with the external iliac veins ventral to the arteries, as in Type I (Fig. 1), and, in addition to this, by means of a single small vessel which lies dorsal to the left common iliac artery. In one individual the caudal vein opened into this dorsal anastomosis (Fig. 12, dorsal view).

Second, as in Figs. 13 and 14, in which the internal iliac veins anastomose ventral to the A. sacralis media (Fig. 14) and then join the external iliac veins to form the postcava by means of four vessels, two of which, the largest, lie ventral (Fig. 14), while the other two lie dorsal (Fig. 13) to the common iliac arteries.

Type III, B, includes those cases in which the principal union between the Yv. iliacae internse and externss takes place dorsal to the arteries in question.

Twenty-four examples of Type 111, B, were met with distributed among nine males and fifteen females.

See Figs. 15, 16, IT, 18, 19, 20, 21, and 22 (ventral views), Plate IV.

Three sets of variations were met with within this Type:

First, as in Figs. 15, 16, 17, 18, and 19, in which the internal iliac veins unite with the external iliac veins to form the postcava (in this case the common iliac veins), by means of three vessels, two of which, the largest, lie dorsal, and one, the smallest, lies ventral to the common iliac arteries.

In Figs. 15 and 16 the internal iliac veins do not anastomose ventral to the A. sacralis media, and the small vessel which lies ventral to the common iliac artery extends between the right (Fig. 15) or left (Fig. 16) internal iliac vein and the common iliac vein of the opposite side. In Fig. 17 the postcava is formed in essentially the same manner as in Figs. 15 and 16, with the exception that the small vessel which lies ventral to the common iliac artery extends, in Fig. 17, between the iliac veins of the same instead of opposite sides. This is a most interesting variation since it represents the persistence, on one side, of the venous ring through which the umbilical artery passes in the embryo (McClure, 02). In Fig. 18 the internal iliac veins anastomose ventral to the A. sacralis media, but in every other respect this variation is similar to that represented by Fig. 16. In Fig. 19 the conditions are the same as in Fig. 18 except that in Fig. 19 the small vessel which lies central to the common iliac artery has changed its position with respect to the iliac veins as the result of a fusion of the common iliac veins.

Second, as in Fig. 20, in which the internal iliac veins anastomose ventral to the A. sacralis media and then unite with the external iliac veins to form the postcava by means of three vessels, two of which, one large and one small, lie dorsal, while one, quite large, lies ventral to the common iliac arteries. This variation differs from the first set only in the circumstance that the single vessel which lies ventral to the common iliac artery is as large as the largest of the two vessels which lie dorsal to this artery.

Third, as in Figs. 21 and 22, in which the internal iliac veins anastomose ventral to the A. sacralis media and then join the external iliac veins to form the postcava (or common iliac veins) by means of ttvo vessels. The larger of these two vessels, which lies dorsal to the common iliac artery, may join the right (Fig. 21) or left (Fig. 22) external iliac vein, while the smaller vessel, which lies ventral to the common iliac artery, then joins the external iliac vein of the o]i]-)osite side.


Type III, C, includes those cases in which the above mentioned dorsal and ventral union are subequally developed.

Eleven examples of this Type Avere met with distributed among two males and nine females.

See Figs. 53, 2-i (ventral views) and 25 (dorsal view), Plate Y.

Two sets of variations ivere met with within this Type :

First, as in Figs. 23 and 24, in which the two internal iliac veins anastomose ventral to the A. sacralis media and join the external iliacs b}^ means of two vessels, subequal in size, which lie respectively dorsal and ventral to the common iliac arteries of opposite sides. In Fig. 23 the ventral vessel joins the right, while in Fig. 24 it joins the left external iliac vein.

Second, as in Fig. 25 (dorsal view), in which the two internal iliac veins, without previously anastomosing ventral to the A. sacralis media join the external iliac vein of the right side by means of two vessels, subequal in size, which lie. respectively, dorsal and ventral to the common iliac artery of the right side.

Tavo Postcaval Variations which cannot be Classed under the

Above Three Types.

In addition to the variations already described in connection with the three Types, two others Avere met with which differ so widely from them in certain fundamental characters that they need a special description.

The main features which characterize these tAvo cases and distinguish them from the variations described under the three Types are tAvofold : (1) All of the posterior tributaries of the postcava unite to form this vessel, as in the higher mammals, dorsal to the arteries; (2) the postcava lies to the left of the aorta instead of upon its ventral surface as in the other cases (Text Figs. YII and YIII).

The position of the ureters in these tAvo cases Avas normal in every respect.

The postcava in these tAvo abnormalities resembles in every detail that of a higher mammal in which the left instead of the right postcardinal vein persists caudal to the kidneys.

The two abnormalities differ from each other only in the manner in Avhich the internal iliac A^eins (Y. i. i.) unite Avjth the external iliacs

'^'The conditions represented by Text Fig. VII are identical to those found in the cat when the left instead of the right postcardinal vein persists as the caudal end of the postcava.




(X.'i.v.). Jn Text Fig. VIT the internal iliac veins (V. i. i.), after anastomosing with each other ventral to the A. sacralis media (A. s. m.), join the external iliac of the left side (V. i. e.) hy means of a common internal iliac vein. In Text Fig. VIII the postcava is partially bifurcated and each int(>rnal iliac vein (V. i. i.) unites with the external iliac (Y. i. e.) of its own side to form a common iliac vein (V. i. c.) without previously anastomosing with the vein of the opposite side, ventral to the A. sacralis media.




Figs. VII and VIII. Two abnormalities of the postrenal division of the postcava of JDldeJpIn/s ■nuivxiipiidix. Ventral views.

A. i.e., A. iliaca communis; A. i.e., A. iliaca externa; A.i.i., A. iliaca interna ; Ao., Aorta; A.s.i.a., A. spermatica interna anterior; A. s.i. p., A. spermatica interna posterior; A.s.m., A. sacralis media; pc, postcava; V.i.c, V. iliaca communis; V.i.e., V. iliaca externa; V.i.i., V. iliaca interna ; V.il., V. iliolumbalis , V.r., V. renalis ; V.s.m., V. sacralis media; V. sp.i., V. spermatica interna.



In view of the circumstance that variations in the mode of origin of the postcava are not exceptional but are the rule in Didelphys, and that these two particular cases differ fundamentally from the usual Types of variation, they must be regarded as the only genuine abnormalities that were met with among the one hundred and one opossums examined.

An explanation of the development of these two abnormalities will be deferred until the second part of this paper. It appears to the writer as w^orthy of mention, however, that the morphological explanation of these two almormalities may be the same as that Avhich accounts for the presence of a postcava in Fciauras taguanoides wliich is similar to that of a higher mammal, as well as for the presence of an allantoic placenta in Perameles.

In addition to the above, another abnormality was met with which, althongh relating to the arterial system, is qnite as remarkable.

As is well known, the posterior mesenteric artery is wanting in all marsupials which have thus far been examined, its place being taken by a branch of the anterior mesenteric artery {Petrogale) or by a branch of the A. coeliacoraesenterica (Didelphys) as the case may be. Among the one hundred and one opossums examined by the Avriter one was met with in which a large posterior mesenteric artery was present, which took the place of the usual posterior mesenteric branch of the A. cceliacomesenterica. In this animal the posterior mesenteric artery (A.m. p.) was given off from the caudal end of the aorta and passed ventrad through a foramen (F.) in the postcaval vein (Text Fig. IX). In its point of origin irom the aorta as well as in its distribution to the intestines this artery agreed in all respects with the posterior mesenteric arteries of other mammals. In addition to its branches to the intestines it also gave off as branches, after passing through the foramen, the two posterior internal spermatic arteries (A. s. i. p.) which were distributed to the ovaries in the usual manner.'*

Owen, 66, has stated that the absence of the posterior mesenteric artery in marsupials is probably related to the simplicity of the mesenteric

'9 In a preliminary paper, 00, tlie writer mentioned the circumstance that two pairs of internal spermatic arteries may spring from the aorta in Didelphyx. It may be stated here that these two pairs of arteries occur with great regularity in Bidelphys and that they were also present in the specimen of PetrogaU recently dissected by the




Fig. IX. Postrenal division of postcava of DidcliJinis niarKupialix in which a foramen is present throug'h which a posterior mesenteric artery passes.

A. i.e., A. iliaca externa; A.m. p., A. mesenterica posterior ; Ao., Aorta ; A.s.i.a , A. spermatica interna anterior; A.s.i.p., A. spermatica interna posterior; F., foramen; L.G., lymph gland ; pc, postcava ; V.i.e., V. iliaca externa ; V.i.i., V. iliaca interna ; V. il., V. iliolumbalis : V.r.,V. renalis; V. sp.i., V. spermatica interna.



attachment of the iutestines. In the writer's estiiiiation its al)sence is more likely related to the circumstance that the postcava in marsupials is closely applied to the ventral surface of the aorta at the point from which the posterior mesenteric artery should arise, and may thereby prove a hinderance to the development of this artery.

Ox THE Presence of a Bifurcated Postcaval Vein in Didelphys.

In about forty-two (42) per cent of the adult opossums examined by the writer, as shown in the following table, the postcaval vein was either bifurcated as far craniad as the internal spermatic veins, or presented some indications of an incomplete fusion between the two vessels which form the postcava caudal to the internal spermatic veins.



Type III.



Type I. Type II. A. B.

6 ? i 6 ? 6 ? 6 V



Postcava bifurcated as far craniad as the Vv. spermaticae



Postcava partially bifurcated caudad of Vv. spermaticye..

Foramen in postcava at level of Vv. spermaticae, and postcava bifurcated caudad of same



Foramen in postcava through which an A. spermatica passes




In twenty-one (21) opossums (8 males and 13 females) the postcava was bifurcated as far craniad as the internal spermatic veins and it is important to note that this is as far craniad as the bifurcation reached



writer (Text Fig. VI). In Didelplu/s the anterior pair of spermatic arteries are not, as a rule, as large as the posterior pair although they may attain this size during the breeding season. In most cases the anterior spermatic arteries do not extend as far as the ovaries or testes but anastomose with the posterior pair of spermatic arteries as represented in Text Fig. V of Petrogale (right side). The two pairs of spermatic arteries are represented in Fig. 33, Plate V and in Text Figs. VI, VII and IX.

in any of the opossums examined. When the postcava was bifurcated as far craniad as the internal spermatic veins one or both of the posterior internal spermatic arteries passed ventrad between the two veins (Figs. 2, 6, 7, 8, 17 and 21, Plates I, II and IV).

In five opossums (3 males and 2 females) the postcava was only partially bifurcated caudad of the internal spermatic veins, as in Figs. 18 and 25, IPlates IV and V).

In nine opossums (1 male and 8 females), as in Figs. 10 and 15, Plates II and IV), a foramen was present in the postcava at the level of the internal spermatic veins through which the posterior internal spermatic arteries passed, and the fusion between the caudal ends of the postcaval veins was incomplete. In one case a posterior mesenteric artery passed through the foramen instead of the spermatic arteries (Text Fig. IX).

In seven opossums (2 males and 5 females) there were no indications of a previous bifurcation except for the presence of a foramen in the postcava, which was situated at the level of the internal spermatic veins, and through which one or both of the posterior internal spermatic arteries passed (Figs. 16 and 20, Plate IV).

In the specimen of Petrogale examined by the writer the left posterior internal spermatic artery (A. s. i. p.) also passed through a foramen in the postcava which, as in Didelphys, was situated at the level of the internal spermatic veins (V. sp. i.) (Text Fig. VI).

The Eelations which Exist Between the Different Types of Variations and the Presence of a Bifurcated Postcava.

As shown by the table, the postcava in the adult was most frequently bifurcated or presented indications of a previous bifurcation, in those animals in which the internal iliac veins make thei.r principal union with the external iliacs to form the postcava by means of vessels which lie dorsal to the common iliac arteries. Thus under Type II, in which the union between the internal and external iliac veins takes place exclusively dorsal to the arteries, twenty cases were met with, and under Type III, B, in which the anastomosis ventral to the arteries is insignificant in character, twelve cases were met with in which the postcava was either bifurcated or presented some indication of a previous bifurcation. Under Type III, C, in which the vessel which lies ventral does not exceed in size that which lies dorsal to the iliac arteries, six cases were met with in which the postcaval veins were incompletely fused caudal to the internal spermatic veins.

Under Types T and III. A, in wliieli the postcava is formed either entirely or for the most part by vessels wliieli unite ventral to the eomnum iliac arteries, only four cases were met with in which it was bifurcated or presented indications of an incomplete fusion.

It appears from the above statistics that we have grounds for assuming that the presence of a double postcava in the adult Didelphys is in some way related to the manner in which the internal iliac veins unite with the external iliacs to form the postcava, since ninety per cent (90^) of the observed cases in which the postcava was bifurcated or presented indications of a previous bifurcation, occurred under Types II, III, B, and III, C, in which the anastomosis between the iliac veins ventral to the iliac arteries is either wanting, or does not exceed in size that which is situated dorsal to the iliac arteries.




Fig. X. Postcava of Echidna embryo No. 44. Ventral view. After Hocbstetter.

Fig. XI. Postcava of Echidna embryo No. 45. Ventral view. After Hocbstetter.

A.i.e., A. iliaca externa; A.i.i., A. iliaca interna; A.m. a., A. mesenterica anterior; A.m. p., A. mesenterica posterior ; Ao., Aorta; A.om., A. ompbalomesenterica ; V.i.e., V. iliaca externa; V.i.i., V. iliaca interna; V.sr., V. suprarenalis.



If this relationship can be definitely established for Didelphys, through further investigation, it would seem to account for the presence of the bifurcated postcava in the adult of Echidna aculeata, since this condition in Echidna is reached through exactly the same venous transformations as those which have apparently produced the bifurcated condition in Didelphys, as can be seen from the following brief account of the development of the postcava in Echidna.

Hocbstetter, 96, who has studied the development of the postcaval vein in Echidna aculeata, finds that, at an early stage, it consists, caudal to the liver, of an unpaired and a paired portion. In a young embryo (J^o. 44, Text Fig. X), the unpaired portion extends from the liver, on the right side of the A. ompbalomesenterica (A. 0. m.), to a point somewhat caudad of this artery. From this point the two postcaval veins (paired portion) extend caudad, at tirst lying ventral to and then on either side of the aorta, and caudal to the A. mesenterica posterior (A.m. p.) anastomose with each other ventral to the aorta. The two internal iliac veins (V. i. i.) which, as Hochstetter says, must be regarded as continuations of the postcaval veins now open into this anastomosis ventral to the common iliac arteries.

In the older embryo (No. 45, Text Fig. XI), the unpaired portion of the postcava possesses the same relations as in embryo No. 44. In the paired portion, however, important changes have taken place. Here the anastomosis between the two postcaval veins caudal to the posterior mesenteric artery (A. m. p.) no longer exists, and the postcava is now formed as in Type III, B, of Didelphijs (Fig. 18, Plate lY), in which the internal iliac veins (V. i. i.) anastomose ventral to the A. sacralis media (A. s. m.) and unite with the external iliacs (Y. i. e.) to form the postcava by means of three vessels two of which lie dorsal and one ventral to the iliac arteries. The transformation of this stage into the adult condition takes place through the atrophy of the vessel which lies ventral to the iliac arteries. The result is that a postcava is formed in Echidna similar to that of Type II in Didelphys, in which the internal iliac veins join the external iliacs dorsal to the common iliac arteries, which is also the Type in Didelphys in which the postcava was found to be most frequently bifurcated.

The Y. pudendovesicalis.

The Y. pudendovesicalis (Y. p. v. in Figs. 1, 8 and 23, Plates I, II and Y) of Didelphys, which returns blood from the bladder and external genital organs, apparently has no regular method of opening into the iliac veins. The remarkable yariations presented by these veins will not be further considered here, however, as they have already been fully described by the writer in a former paper (jMcClure, oo, p. 457) to which the reader is referred.

==Literature==

1895. Beddard, F. E. — On the Visceral Anatomy and Brain of Dendrolagus berinitte. Proc. of Zool. Soc. of London for 189,5.

1902. Beddard, F. E. — IMammalia.

1882. CuiwriNGHAM, D. J. — Challenger Reports, Zoology, Vol. V.

1898. Devez, G. — Une point d'Anatomie du ventricule droit des Didelphys. Bull. ]\r\is. Hist. Nat., Paris, Annee, 1898.

1881. Forbes, W. A. — On some points in the Anatomy of the Koala (Phascolarctos einrreiis). Proc. of Zool. Soc. of London for 1881.

1893. HOCHSTETTER, F. — Beitriige zur Entwickelungsge.schichte des Venensystems cter Amnioten, III. Sanger. Morph. Jahrb., Band II.

1S96. HocHSTETTER, F. — " Monotremen und Marsupialier " in Senion's Zoologische Forschungsreisen in Australien. Band II, Lief. III.

1830. Martin. — On the Anatomy of the Koala. Proc. of Zool. Soc. of London for 1836.

1900. McClure, C. F. W. — The Variations of the Venous System in Didel phys viryinkinu. Anat. Anz., Band 18.

1901. McClure, C. F. W. — The Anatomy and Development of the Posterior

Vena Cava in Didelphj^s virginiana. Zool. Bulletin, Vol. II, No. 6.

1902. McClure, C. F. W. — The Development of the Postcaval Vein in Didel phys virginiana. Science, N. S. Vol. XV, No. 379. 1876. NuHN. — Keference taken from page 414 of Henle's Gefasslehre des Menschen, 1876.

1835. Owen, E. — Note on Dasynrus nuicrtiriis. Proc. of Zool. Soc. of Lon don for 1835.

1836. OwEfN, R. — Notes on the Anatomy of the Wombat. Proc. of Zool. Soc. of Loncion for 1836.

1839-1847. Owen, E. — ■" Marsupialia " in Cyclopaedia of Anatomy and Physiology, Vol. III.

1866. Owen, E. — Comparative Anatomj' and Physiology of Vertebrates, Vol. III.

1896. Parsons. — On the Anatomy of Petrogale xantJwpus compared with that of other Kangaroos. Proc. of Zool. Soc. of London for 1896.

1890. EoSE, (i. — ^BcitrJige zur vergl. Anatomic des Herzens der Wirbelthiere. Morphol. Jahrb., Band XVI.

1902. WiEDERSHElM, E. — Grundriss der vergleichenden Anatomic.

1898. WiNDLE and Parsons. — On the Anatomy of Macropus nifu.s. Journal of Anatomy and Physiologj-, Vol. 32.

==Explanation of Plates==

All of the Figures on Plates I to V are life sized di-awings of the caudal end of the postcava of Didelphys marstipialis and illustrate the variable manner in which the postcava is formed in this animal. With the exception of Fig. 23 only that portion of the postcava is represented which lies caudal to the Vv. spermaticae internae. In Fig. 23 the entire " postrenal division " of the postcava is represented. The names of all the vessels are indicated on three Figures (Fig. 1, Type I: Fig. 8, Type II, and Fig. 23, Tyj)e III, C, which can be used in identifying the vessels in the other Figs. Fig. 3 is only partially lettered).

The following abbreviations apj)iy to the lettering on Figs. 1, 3, 8 and 23:

(d and s refer to dextra and sinistra, respectively).

A. i. c, A. iliaca communis.

A. i. e., A. iliaca externa.

A. i. i., A. iliaca interna.

Ao., Aorta.

A. s. i. a., A. spermatica interna anterior.

A. s. i. p., A. spermatica interna posterior.

A. s. m., A. sacralis media.

bl., Accessory branch of the A. spermatica interna posterior to the bladder.

Pc, Postcava.

P. E., Postrenal Division of Postcava.

ps., Accessory brnnch of the A. spermatica interna posterior to tlie psoas muscle.

V. i. c, V. iliaca communis.

V. i. e., V, iliaca externa.

V. i. i., V. iliaca interna.

V. i. i. c, V. iliaca interna communis.

V. i. 1., V. iliolumbalis.

V. p. v., V. pudendovesicalis.

V. r., V. renalis.

V. s. m., V. sacralis media.

V. sp. i., V. spermatica interna.

Plate I. — Type I.

Fig. 1. Ventral view {^).

Fig. 2. Ventral view (^P). Postcava bifurcated caudad of

V. spermatica interna sinistra. Fig. 3. Ventral view (^p). An accessory branch of the A.

spermatica interna posterior (A. s. i. p.) is represented in

this Fig. which supplies the psoas muscle (ps.) and bladder

(bl.). Fig. 4. Ventral view i(^). An anastomosis is present on

the right side between the V. spermatica interna and the

V. pudendovesicalis. Fig. 5. Dorsal view (.p).

===Plate II===
Type II.

Fig. 6. Ventral view (^). Postcava bifurcated caudad of Vv. spermaticae. Postcava formed here as in Echidna aculeata.

Fig. 7. Ventral view (^). Postcava bifurcated caudad of Vv. spermaticae internae.

Fig. 8. Ventral view ((^). Postcava bifurcated caudad of Vv. spermaticae internae.

Fig. 9. Ventral view (^P).

Fig. 10. Ventral view ((^). Foramen in postcava through which the Aa. spermaticae internae posteriores pass. Postcava partially bifurcated at its caudal end. On account of the variable character of the V. pudendovesicalis this vein which is connected, on the right side, with the V. iliaca externa and the V. iliaca interna communis has not been regarded as entering into the formation of the postcava.


===Plate III===
Type 111, A.

Fig. 11. Dorsal view (,p). Fig. 12. Dorsal view {(^). Fig. 13. Dorsal view of Fig. 14. Fig. 14. Ventral view d^).

Plate IV.— Type III, B.

Fig. 15. Ventral view (^p). Aa. spermaticae Internae posteriores pass through a foramen in postcava.

Fig. 16. Ventral view {^). Aa. spermaticae internae posteriores pass through foramen in postcava.

Fig. 17. Ventral view (^P). Postcava bifurcated caudadof Vv. spermaticae internae. Remains of venous ring through which the umbilical artery passes in the embryo.

Fig. 18. Ventral view (,p). Postcava partially bifurcated at its caudal end.

Fig. 19. Ventral view {(^).

Fig. 20. Ventral view ((J*). Foramen in postcava through which the Aa. spermaticae internae posteriores pass.

Fig. 21. Ventral view i^P). Postcava bifurcated.

Fig. 22. Ventral view ((^).

Plate V.— Type III, C.

Fig. 23. Ventral view i^). Postrenal division of postcava;

also showing the two jiairs of internal spermatic arteries

(A. s. i. a. and A. s. i. p.). Fig. 24. Ventral view (^). Fig. 25. Dorsal view ij^)- Postcava partially bifurcated caudad of the Vv. spermaticae internae.