OF DIDELPHYS VIRGINIANA. 73 



transverse and a longitudinal direction, begins, and increases to the last. With the cessa- 

 tion of the articular "tubercles" for the ribs, the pre-zygapophyses begin to be sur- 

 mounted by an elongated, pointed process, which grows less acute, and at the same time 

 broader and heavier with succeeding vertebrae. The neural arches are low, broad, and 

 comparatively thin, in the anterior portion of the dorsal series ; they grow higher, longer, 

 and more massive with each successive vertebra, and the same may be said, in general 

 terms, of the centrums themselves. The intervertebral foramina are largest anteriorly ; 

 they progressively decrease in size from before backward, and at the same time gradually 

 slide from the directly lateral position they occupy at first, more to the front of the col- 

 umn, so that they look forward as well as outward. The body of the first dorsal has one 

 full costal facet, the others two demifacets, the rib being implanted on the intervertebral 

 fibro-cartilage, and only nicking the edge of the bone itself The bodies of the middle 

 vertebrae are perfectly smooth on their thoracic aspects, and very convex from side to side ; 

 those at either extremity are slightly ridged by rudimentary hypapopliyses. 



Lumbar Vertchrm. — (Fig. 15.) The first lumbar repeats the characters of the last dor- 

 sal, and there is little modification in the remaining five bones of the series. The fourth 



vertebra, counting from before back- ^_____, 



ward, is the largest of the set ; the ^—^I^^^^^^^Zl^^^^\ 

 bones diminish in size in both direc- cJllP^^t^w^ 

 tions from this one. The flexibility of 

 this part of the spine is considerable in 

 every direction ; the movement of ex- 



, . , , ■ 1 J T • 1 , -^ig- 15. — Lumbar Vertebrae. 



tension beyond a right . line is least ; 



that of flexion greatest. Few animals, indeed, except man, can make the " small of the 

 back " concave. The zygapophyses bear nearly vertical articulating facets ; the post- are 

 locked within the pre-zygapophyses. The two processes are on either side connected by 

 an acute, longitudinal ridge, which divides the side of the column into a narrow, deep 

 sulcus above, between itself and the spinous processes, and a rather broad, shallow groove 

 between itself and the series of transverse processes ; these grooves are filled by the two 

 divisions of the muscles of the loins. As already hinted, the transverse processes 

 are greatly expanded lengthwise, in a slightly oblique direction, and mutually overlap 

 each other like those of the neck. They increase in size and obliquity from first to last. 

 The spines of the vertebrae are lowest and thickest at the first ; they grow longer, higher, 

 and thinner to the fifth ; this and the sixth retain the height and thinness of the spine, 

 but the length diminishes. The bodies of these vertebrae are, upon their ventral aspects, 

 very convex transversely, and moderately concave lengthwise. The massiveness of the 

 lumbar vertebrae, and the extent of their several processes, correspond with the great size 

 and strength of the lumbar muscles, whose power must be sufficient, with the aid of the 

 abdominal muscles, to enable the animal to flex the body to regain an ordinary position 

 when suspended by the tail. Origins of caudal muscles, moreover, can be traced past the 

 sacrum into this region of the spine, in which is their initial force. 



Sacral Ferieferce.— (Fig. 16.) If the iliac synchondrosis be held to define the sacrum, this 

 is composed of two bones ; if serial anchylosis be taken as a guide, there are three sacral 

 vertebrae. In adult animals, the equivocal bone is firmly united to the one preceding ; but, 

 at the same time, its physical characters are those of several succeeding coccygeals — as one 



MEMOIRS B03T. 80C. NAT. H13T. VOL. II. 19 



