284 



MARSUPIALIA. 



Fig. 110. 



fetuses that we ought to find auxiliary parts 

 for increasing the power of the muscles engaged 

 in parturition. The bones in question are, 

 moreover, equally developed in both sexes : 

 and they are so situated and attached that they 

 add to the power of the muscles which wind 

 round them, and not of those implanted in 

 then). They are not, however, merely sub- 

 servient to add force to the action of the " cre- 

 masteres," but give origin to a great proportion 

 of the so-called " pyramidales." 



The osteogencsis of the marsupial pelvis de- 

 rives some extrinsic interest from the not yet 

 forgotten speculations which have been broached 

 regarding the analogies of the marsupial bones. 

 These have been conjectured to exist in many 

 of the placental Mammalia, with a certain la- 

 titude of altered place and form, disguised, 

 e. g. as the bone of the penis in the Carnivora, 

 or appearing as the supplemental ossicles of 

 the acetabulum, which exist in the young of 

 many of the Rodentia. In the os innomina- 

 tum of the immature Potoroo the curved pris- 

 matic ilium contributes to form, by the outer 

 part of its base, the upper or anterior third of 

 the acetabulum; the rest of the circumference 

 of this cavity is completed by the ischium and 

 pubis, excepting a small part of the under or 

 mesial margin, which is formed by a distinct 

 ossicle or epiphysis of the ilium, (a, jig. 1 10,) 



antilogous to that de- 

 scribed by Geoffrey St. 

 Hilaireastherudimen- 

 tal marsupial bone in 

 the rabbit. Now here 

 there is a co-existing 

 marsupial bone : but 

 besides the five sepa- 

 rate bones just men- 

 tioned, there is a sixth 

 distinct triangular ossi- 

 cle, which is wedged 

 into the posterior in- 

 terspace of the ischio- 

 pubicsymphysis. How 

 easy were it to suggest 

 that this single sym- 

 metrical bone may be 

 the representative of 

 the os penis removed 

 from the glans to the 

 root of the intromittent 

 organ ! 1 regard it as 

 a mere epiphysis of the 

 ischium. The circumference of the acetabulum 

 is always interrupted by a deep notch opposite 

 the obturator foramen, which is traversed by 

 a ligamentous bridge, and gives passage to 

 the vessels of the Harderian gland lodged in 

 the wide and deep acetabular fossa. 



The penial like the marsupial bone is essen- 

 tially an ossification of the fibrous or sclerous 

 tissue. 



The femur is a straight, or nearly straight, 

 long, cylindrical bone, having a hemispherical 

 head supported on a very short neck, espe- 

 cially in the Petaurists, and situated here 

 almost in the axis of the shaft, above and 

 between the two trochanters, which are nearly 



of equal size. In the Kangaroos and Potoroos 

 the head of the thigh-bone is turned more 

 inwards, and the outer or greater trochanter 

 rises above it. In other Marsupials the great 

 trochanter is less developed. In most of the 

 species a strong ridge is continued downwards 

 to within a short distance from the trochanter, 

 and this ridge is so produced at the lower part 

 in the Wombat as almost to merit the name 

 of a third trochanter. In the Wombat and 

 Koala there is no depression for a ligamentum 

 teres. The shaft of the bone presents no lima, 

 usperte. 



The canal for the nutrient artery commences 

 at the upper third and posterior part of the 

 bone in the Koala, and extends downwards, 

 contrariwise to that in most other marsupial 

 and placental Mammalia. 



At the distal extremity of the femur the 

 external condyle is the largest, the internal 

 rather the longest. The intermediate anterior 

 groove for the patella is well marked in the 

 Perameles, where the patella is fully developed, 

 but is broad and very shallow in the Phalan- 

 gers and Dasyures, where the tendon of the 

 rectus muscle is merely thickened or offers 

 only a few irregular specks of ossification; and 

 the corresponding surface in the Petaurists, 

 Wombat, and Koala is almost plane from side 

 to side; in these Marsupials and in the Myrme- 

 cobius the patella is wanting. I find a distinct 

 but small bony patella in the Macropus Ben- 

 nettii. There is a sesamoid bone above and 

 behind the external condyle of the femur in the 

 Myrmecobius and some other Marsupials. 



In the knee-joint, besides the two crucial 

 ligaments continued from the posterior angles 

 or cresses of the semilunar cartilages one to 

 the outer side of the inner condyle, the other 

 to the interspace of the condyles there is a 

 strong ligament which passes from the anterior 

 part of the tibial protuberance backwards to 

 the inner side of the fibular condyle, and a 

 second continued from the same point along 

 the outer margin of the outer semilunar car- 

 tilage to the head of the tibia. 



The tibia (a, Jig. Ill) presents the usual dis- 

 position of the articular surface for the condyles 

 of the femur, but in some genera, as the Wombat 

 and Koala, the outer articular surface is continu- 

 ous with that for the head of the fibula. In the 

 Kangaroos and Potoroos the anterior part of 

 the bead is much produced, and in the young 

 animal its ossification commences by a centre 

 distinct from the ordinary proximal epiphysis 

 of the bone. A strong ridge is continued 

 down from this protuberance for about one- 

 sixth the length of the tibia. In the Koala a 

 strong tuberosity projects from the anterior part 

 of the tibia at the junction of the upper with 

 the middle third. In this species and in the 

 Wombat, as also in the Opossums, Dasyures, 

 Phalangers, and Petaurists, the shaft of the 

 tibia is somewhat compressed and twisted ; 

 but in the Kangaroos, Potoroos, and Pera- 

 meles the tibia is prismatic above and sub- 

 cylindrical below. The internal malleolus is 

 very slightly produced in any Marsupial, but 

 most so in the Wombat. 



