162 



PELVIS. 



still further in the Insectivora and Cheiroptera. 

 The increasing obliquity of the pubes also 

 indicates an approach to the Bird type. 



The climbing habits of the Sloths produce a 

 habitual vertical position of the trunk, re- 

 quiring for the support of the abdominal 

 viscera large open pelves. 



In the Ai {Bradyptu tridactylus) the pelvis 

 {fig. 100.) is remarkably slender, expanded, 

 shallow and horizontal in direction, the pelvic 

 openings being very large and round, and the 

 antero-posterior diameters little larger than the 

 transverse. The ankylosis of the innominate 

 bones to the sacrum in these animals gives a 

 great firmness to the support of the otherwise 

 feeble hinder extremities, and with the great 

 distance of separation of the acetabula, which 

 are small and shallow, assists to a considerable 

 degree their climbing and holding powers, and 

 to produce that slowness and awkwardness of 

 motion which has given them the name of 

 Tardigrades. 



Fig. 100. 



Pelvis of the Ai, anterior view. 



The sacrum is large, both in length and 

 breadth, very flat, with large, open foramina, 

 and presenting a flattened surface in place of 

 the posterior spines and tuberosities. It is 

 composed of five vertebrae, of which the three 

 upper (e) as well as the last lumbar (g) are 

 ankylosed to the ilia (&). The union of the 

 last lumbar seems to result from an extension 

 of ossification in the ilio-lumbar ligament 

 (0, and contributes much to increase the 

 steadiness of the spinal column on the pelvis. 

 The coccyx is triangular, little curved, broad 

 and short, and is composed of six pieces. In 

 some species it is prolonged into a tail* 



The ilia are short and slender, with much- 

 expanded wings, having an anterior concavity 

 and a plane surface posteriorly. They are 

 ossified to the sacrum at an early period. 

 The ischia are short and slender, and united 

 to the last sacral vertebra, and more slightly 

 to the two above it, by ossification of the 

 great sacro- sciatic ligament (), which gives to 

 the angle of the bone an expanded appear- 

 ance, and encloses a round, wide sacro-sciatic 

 foramen (d), above and behind the cotyloid 

 cavity. The tuberosities are small, and the 



inferior rami (/) are long and slender, enclosing 

 with the pubis a very large obturator foramen, 

 having its long diameter from side to side, and 

 do not join in symphysis. The pubes (//) are 

 long and slender, their rami united in a V shape, 

 with the angles meeting to form a very short 

 symphysis (c), which is sometimes ossified, and 

 presenting a very slight ilio-pectineal spine (i). 

 The lumbo-Hiac angle in the adult Bradypus is 

 about 145, and the ilio-pubic about 155, 

 being only about 25 from a right line as in 

 the human pelvis. The ilio-ischial angle also 

 approaches the human standard in being di- 

 minished to 135. 



This diminution of the ilio-ischial angle 

 is still more remarkably shown in the My- 

 lodon and Megatherium fossil gigantic Sloths, 

 which approach more closely to Man in this 

 respect than any other Mammalian. 



The osseous system of the fossil Mytodon 

 robustus closely resembles that of the Sloths, 

 differing from thenr, however, by presenting a 

 continued sacral crest, and more expanded ilia 

 {fig. 101.). According to Professor Owen, in 

 his valuable monograph on the specimen in the 

 Hunterian Museum, the sacrum really con- 

 sists of seven vertebras, but by ankylosis with 

 the three lumbar and last dorsal includes 

 eleven vertebrae, and forms one strong and con- 

 tinuous bony mass along the whole lumbar re- 

 gion (a). Its total length is 2 feet 4 inches, 

 and it gradually increases in breadth to the 

 sacro-iliac union (<?), which is formed by the 

 first, second, and third true sacral vertebras, and 

 there presents its greatest breadth. It then 

 contracts slightly, and, at the sixth and last, ex- 

 pands again to join the ischia (d). It is firmly 

 united by ankylosis both to the ilia and ischia. 

 Its anterior surface is curved both laterally 

 and vertically. The spinal canal is very wide, 

 and the foramina passing from it mark the 

 primary vertebral divisions. The whole of 

 the eleven spinous processes of the ankylosed 

 vertebras form a remarkable curved crest pos- 

 teriorly (g). There are twenty-one caudal ver- 

 tebras, which doubtless, in the living animal, 

 contributed to support the body by applica- 

 tion to the limbs of the trees upon which it 

 climbed, and were strongly supported by the 

 sacro-sciatic ossification. 



The innominate bones are very large. The 

 iliac wings are much spread out, widety con- 

 cave anteriorly, and slightly convex posteriorly, 

 these surfaces being directed forwards and 

 backwards. The iliac crest presents a con- 

 tinuous, well-arched curve, and at the inner 

 part of its centre (m) it is prolonged and 

 curved forward in a lip which overhangs the 

 anterior fossa, and contributes to support 

 the bulky viscera. The lips of the crest are 

 remarkably spread. The posterior superior 

 spine is continued by an oblique crest of bone 

 (a) to the lateral tubercles of the lower 

 sacral vertebras, as if from ossification of the 

 oblique sacro-iliac ligament. The posterior 

 inferior spines are continued by a ridge to the 

 borders of the fifth, sixth, and seventh sacral 

 pieces into the bony ankylosis of these with 

 the ischia, which are ankylosed to the same 



