OF DIDELPHYS VIRGINIANA. 103 



and is attached to the salient points of all the vertebrae. The series of septa that it forms 

 between the V-shaped bones, separating the two principal flexing bundles, is the strongest. 

 The long tendons above referred to lie embedded in this fibrous sheath, and are thus 

 plaited, as it were, into stout, flattened bands. These bands are four in number, two above, 

 and two below, on each side of the median line ; the tendons of each are from twelve to 

 twenty in number, and may be distinctly seen without dissecting, lying close beside each 

 other, most of them running the whole length of the tail, but some ending by attach- 

 ment to nearer vertebrae. In general effect, these bands of tendons call to mind the con- 

 joined tendon of a digital flexor, befoi-e the filaments become separate from the common 

 bundle. Each tendon arises from a muscular fasciculus in the sacral or pelvic region, as 

 will be presently noticed. Besides encasing the tail, the fibrous sheath gives origin, by its 

 under surface, to muscular fasciculi, particularly along the course of the tendon-bands, which 

 cannot be raised from the subjacent muscle without laceration of the latter. 



On either side of the median line, on the superior aspect of the tail, a rather small mul- 

 tifid muscle occupies the groove between the spinous and articular processes. This is the 

 backward continuation, unbroken, of the series of slips that fills the same groove in the 

 lumbar region, and it has precisely the same characters. Fasciculi constantly arise from 

 successive vertebrae, and blend externally and superiorly into a continuous band. These 

 slips are, or rather this muscle is, distinct, as far as the sixth — eighth caudal ; there it 

 virtually ends, partly by insertion by separate tendons into the proximal extremities of 

 the vertebrae, but chiefly by attachment to the under surflice of the tendon band. The few 

 fleshy fibres that may be traced nearly to the tip of the tail, on the under surface of this 

 band, between its successive attachments to the vertebrae, may properly be regarded as 

 the continuation of this muscle. 



The chief extensor muscle of the tail lies next to the preceding, on the outer side of it, 

 as far as its muscular part extends ; above it, along its tendinous portion. This is the 

 muscle that forms the bundle of long tendons. It arises high up in the lumbar region, 

 where it forms a distinct belly lying in the groove between the articular and transverse 

 processes, covered by the aponeurotic origin of the erector spinae, and behind, or to the 

 median side of, the quadratus lumborum. In the sacral and upper caudal region it com- 

 pletely fills the groove — at first between the articular processes and the ilium, subsequently 

 between the same processes and the transverse. It consists of a large number of fasciculi, 

 taking fleshy origin from successive vertebrae at the very bottom of the fossa, and each 

 ending in a long, slender tendon. The course and arrangement of these tendons has 

 already been noticed. The ftisciculi that form the tendons may all be easily dissected apart 

 to their very bases, and form in eftect so many small muscles. Most of them become ten- 

 dinous at nearly the same point — opposite, or a little beyond, the tuberosity of the ischia. 

 Two or three of the most posterior of them, however, continue fleshy for a greater distance. 



To the oiiter side of the foregoing lies another smaller muscle, properly belonging to the 

 extensor series, but really subservient to the lateral movements of the tail. It represents 

 the continuation, behind the ilium, of the most lateral or ventral series of the lumbar mus- 

 cles. It arises fleshy from the transverse processes of the sacrum, and corresponding as- 

 pect of the ilium, and forms a small, continuous bundle, situated upon the very apices of 

 the successive transverse processes as long as the latter continue to be developed. 



