File:The cyclopædia of anatomy and physiology (1859) (20635101799).jpg

Title: The cyclopædia of anatomy and physiology
Identifier: cyclopdiaofana05todd (find matches)
Year: 1859 (1850s)
Authors: Todd, Robert Bentley, 1809-1860
Subjects: Anatomy; Physiology; Zoology
Publisher: London, Sherwood, Gilbert, and Piper
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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PELVIS. 161 (/) in the Kangaroo, Phascogale, and Potoroo, is not parallel with the spinal column, but Fig. 99.
Text Appearing After Image:
Pelvis of the Kangaroo, showing the marsupial bones (a «) and inter-sciatic eplphysis (c). oblique in the opposite direction to the human symphysis, so that if prolonged forwards the line of direction would cut the spinal column at an obtuse angle. This makes the posterior opening larger in its antero-posterior diameter than it otherwise would be, and allows for the great encroachment of the caudal ver- tebra posteriorly. The sciatic notch is long and narrow, corresponding to the great length of theischia ; and the foramen obturatorium is large and elongated antero-posteriorly from the same cause. In the Daxynrus and Pe- tnunsts, the ischio-pubic symphysis is oblique in the opposite direction. The antero-pos- terior diameter of the anterior outlet in the Kangaroo is greater than the transverse by about half an inch ; but at the posterior outlet, the transverse is a little greater, from the pro- jection of the caudal spines before mentioned. The pelvic cavity is deep in the Marsupialia, and its openings are small in proportion to the size of the animal, since the foetus is expelled before it is full grown, and placed on the nipples in the marsupial pouch to complete its deve- lopment into a state of independent existence. But the proportion between the pelvic open- ings and the size of the foetal head, at the period of expulsion, is very remote. Even in the Petaurists, whose pelves are the smallest, the cavity and openings are six times the size of the foetal head. The muscles of the tail and "legs attached to the pelvis are, in the Kangaroos, very powerful to perform their prodigious leaps, especially the gracilis and biceps. The glutei, however, are not large, since the trunk is not held erect on the legs by these muscles, but is suspended, as it were, between the femurs, and supported in front by the largely developed psoae muscles, and behind by the powerful tail, used as a propelling organ by the sudden action of its flexor muscles. The pelvis of the Monotremata resembles in general appearance the reptile type, although Supp. in some other respects these curious animals, especially the Ornithorhyncus, approach the Birds. The sacrum of the Ornithorhyncus is composed of two vertebrae, separated, as in the Saurian reptiles, and placed in the line of the lumbar curve, differing little in appearance from the lumbar vertebras. In the Echidna are three sacral vertebrae, also separated and all uniting with the ilia. The ilia are short, thick, and prismatic, and project above the spine at an angle of 140° as high as the sacral spines, and presenting, in the Ornithorhyncus, considerable eversion of the alas, and, in a much less degree, in the Echidna also. The ischia are short, bent upwards in the former, and project backwards at the tuberosi- ties in an angular spine, most marked in the Ornithorhyncus, and giving a reptile-like ap- pearance. The pubes are broad and short, placed at a marked angle with the ilia, 110° in the Echidna hystrix and 120° in the Ornitho- rhyncus, and uniting by broad plate-like rami with the ischial rami, which form with them a long ischio-pubic symphysis. The ischio- pubic plate thus formed is very like that seen in the reptiles. The marsupial bones are also present, and are very large and strong in this class, although not provided with a pouch. In the Ornitho- rhyncus they are broad and triangular, articu- lated by the base to the whole length of the pubic crest meeting in the median line, and with their rounded apices directed forwards and outwards. In the Echidna they are longer, rounder, more pointed and less everted, with two articular processes at the pubic extremity (see Jig. 177. Art. Monolrcmata). The ilio-pectineal spines are also very large in the Ornithorhyncus, and in a less degree in the Echidna. The obturator foramina are small. The three pelvic bones are united at the cotyloids by bony union in the Ornithorhyncus. In the Echidna hystrix, the union of these bones is, however, effected by cartilage only, and the acetabnla are perforated by a consider- able opening into the pelvic cavity,constituting another remarkable reptile-like peculiarity. Having traced the Mammalian pelvis to a form presenting somewhat of the reptile tvpe in the Monotremes, we may now recur back to an order of animals which, from their general organisation, are connected closely to the order of primates, and are usually placed much higher in the animal scale than the position here assigned to them. These are the Sloths or Tardigrades, which form the connecting link between \heSimiee and Edentata proper. Their pelvic peculiarities, however, ally them more closely to the Birds. The moststrilung of these is the ossification of the ilia and ischia to the broad sacrum, by transformation of the sacro-iliac and sacro- sciatic ligaments. We have already noticed an exceptional example of this coalescence in the Ruminants, in the Alcminna or pigmy Chevrotain. But the pelvis of the Edentata also presents a diminution of the pubic sym- physis, and the absence of the ischia from this junction, a separation which is carried M

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