MONOTREMATA. 



375 



following vertebrae support long and functional 

 ribs, so that there are three lumbar vertebrae in 

 the Echidna, and but two in the Ornithorhyn- 

 chus, which thus resembles the Lizards in the 

 great proportion of the trunk which is encom- 

 passed by the costal arches. Another approxi- 

 mation to the Oviparous type is made by the 

 long-continued separate state of the short 

 cervical ribs in both Monotremes : these in a 

 young but nearly full-grown Echidna are 

 detached from all the cervical vertebrae except 

 the atlas. The vertebral end of the cervical 

 rib is bifurcated ; the lower branch, represent- 

 ing the head, is articulated to the transverse 

 process or tubercle developed from the body 

 of the vertebra; the upper branch, represent- 

 ing the costal tubercle, is articulated to a trans- 

 verse process developed from the side of the 

 base of the neural arch. In the Ornitho- 

 rhynchus the cervical ribs appear to become 

 earlier anchylosed to the vertebrae, except as 

 regards the axis, in which the broad costal 

 appendage retains its original independence 

 throughout life, and is slightly moveable 

 upon the confluent extremities of the two 

 transverse processes. In the succeeding ver- 

 tebrae the space intercepted between the two 

 transverse processes, the vertebra, and the cos- 

 tal rib, forms the so-called * perforation of the 

 transverse process' for the vertebral artery in 

 human anatomy. In the Echidna, above alluded 

 to, the ' neurapophyses' or vertebral plates of 

 the atlas, which together form the neural or 

 spinal arch, were unanchylosed at their upper 

 or spinal extremities. The atlas of the Orni- 

 thorhynchus is chiefly distinguished from that 

 of the Echidna by the continuation, from the 

 lower part of its slender body, of two long 

 diverging processes which are developed in the 

 strong tendons of the recti capitis antici 

 muscles. The spine of the dentata is broad 

 and high in both Monotremes ; those of the 

 other cervicals progressively diminish in size 

 in the Ornithorhynchus, but become at once 

 nearly obsolete in the Echidna. The transverse 

 and spinous processes are of moderate size in 

 the rest of the true vertebrae, but are largest in 

 the lumbar region. The posterior oblique 

 processes are single in these vertebra. The 

 articular surfaces of the vertebrae, which are 

 slightly concave, are joined together by a thick 

 circular band of ligamentous fibres (fig. 174, a) 

 attached to the circum- 

 ference of the articular 

 surface, enclosing a cen- 

 tral oblate spheroidal ca- 

 vity (6) lined by a synovial 

 membrane and filled with 

 fluid. 



The ribs are long and 

 slender in the Ornitho- 

 rhynchus, somewhat stron- 

 ger in the Echidna. The 

 first is flattened, the rest 

 are cylindrical. Each rib 

 is articulated by a single 

 joint, uniting the head to 



the vertebral interspace, Intervertebral camties, 

 or to the side of the cen- Echidna. (Oriyinal.) 



Fig. 174. 



trum, as in the two last pairs: the tubercle, 

 though small, is distinctly developed, and de- 

 fines the neck of the rib, although it does not 

 join the transverse process of the vertebra. 



Meckel's statement, ' tuberculum adest nul- 

 lum,' is applicable only to the last three or 

 four pairs of ribs. The first six pairs are joined 

 to the sternum in both the Ornithorhynchus and 

 the Echidna. Six pairs of ossified sternal ribs 

 (hamapophyses*) are articulated to the ster- 

 'num in the Echidna ; the first four are nearly 

 straight and sub-cylindrical ; the fifth and sixth 

 are expanded. Five pairs of ossified sternal 

 ribs are present in the Ornithorhynchus, to 

 which the second to the sixth vertebral ribs 

 inclusive are joined by shorter intervening 

 cartilaginous pieces of a similar form. The 

 first rib in the Ornithorhynchus is joined to the 

 sternum by cartilage alone. The interposed 

 cartilages, which thus form a third element in 

 the costal arch, repeat a structure common in 

 crocodiles, and may be regarded as the homo- 

 logues of the costal appendages in the ribs of 

 birds, which in this class are removed from the 

 interspace of the vertebral and sternal rib, and 

 articulated to the vertebral piece. 



The cartilages of the false ribs in both genera 

 are singularly expanded and flattened, and 

 present an imbricated arrangement, gliding 

 upon each other with a slight yielding motion : 

 the last vertebral rib, which is the shortest and 

 straightest, is terminated by a short free carti- 

 lage. Many bone-corpuscles are scattered 

 through these cartilages. 



The ordinary sternum, to which the true ribs 

 articulate, consists of four ossicles, and in the 

 Echidna a fifth is developed in the base of the 

 ensiform cartilage ; the most anterior of these 

 ossicles (fig- 173, A, s) has the usual expanded 

 hexagonal form and large proportions of the 

 manubrium sterni, and receives, besides the 

 first and part of the second pair of ribs, the 

 extremities also of the coracoid bones. The 

 ensiform cartilage in the Echidna presents an 

 elongated oval form with a central perforation. 

 A large T-shaped episternal bone (Jig. 173, A, f) 

 is articulated to the anterior surface of the 

 manubrium sterni; it is the key-bone of the 

 complicated scapular arch. 



The swum consists in the Ornithorhynchus, 

 as in most Saurians, of two vertebrae, distin- 

 guished by the greater breadth and thickness 

 of their transverse costal processes. In the 

 Echidna there are three sacral vertebrae. 



There are thirteen caudal vertebrae in the 

 Echidna (Jig. 168). The first is the largest, 

 with broad transverse processes, the rest pro- 

 gressively diminishing, and reduced, in the six 

 last, to the central element. The Ornithorhyn- 

 chus (Jig- 173, A) has twenty-one caudal ver- 

 tebrae, of which all but the last two have 



* In the complete vertebrae which encompass 

 the centre of the vascular system, the haemapo- 

 physes or halves of the chevron-bone, or inferior 

 vertebral laminae, are retained, together with the 

 inferior or haemal spine. The series, sometimes 

 anchylosed, as in Man, of these spines forms the 

 ' sternum :' the inferior lamina; or haemapophyses 

 are the sternal ribs. 



