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Show 1890.] HELODERMA SUSPECTUM. 177 Of the Musculature of the Trunk and Tail. 56. Spinalis dorsi.-Heloderma suspectum has this muscle quite powerfully developed, it being a firm, longitudinal welt wedged in between the neural spines of the vertebra, on the one hand and the longissimus dorsi muscle on the other, and extending the entire length of the back. Its thickest parts are in the cervical and dorsal regions, while down the latter half of the tail it gradually tapers away to a tendinous thread at the tip. Its structure is well seen in the mid-dorsal region, where superficially it is characterized by a series of oblique, closely juxtaposed, tendons, which, passing forward from the muscular mass, and stretching by nearly four of the vertebrae, are each in turn inserted into a neural spine of one of the same. Still more deeply situate we find other tendons somewhat similar to these last, which are inserted into the interspinous ligaments, the fascia, and more or less upon the sides of the neurapophyses themselves. All these I take to be tendons of insertion of the spinalis dorsi, and cutting down more deeply on the muscle we find its origin to be a system of tendons which arise from the anterior margins of the prezygapophyses of the vertebrae and by fleshy origins from the superior aspects of the same. Where the muscle passes over the pelvis, corresponding attachments are made to the sacral vertebrae. Following it into the cervical region, we find the spinalis dorsi still thick though more laterally compressed, and it is finally inserted, first by a tendon, having something of the character of a ligamentum nuchce, into the middle of the posterior border of the parietal bone, mesiad to the complexus, into the supraoccipital which the latter overhangs, and also by stout carneous fasciculi into the posterior margins of the neurapophysis, the postzygapophysis, aud to some sligbt extent into the ventral surface of the atlas vertebra. These insertions are not entirely fleshy, but semitendinous, and the neural spine of the atlas is much aborted. As we pass from sacrum to tip of tail the spinalis dorsi, as I have already said, gradually diminishes in size, while at the same time it comes to be more and more intimately blended with the supracaudal upon either side of it, as it is between these muscles that it is found in this part of its course. The muscles of the nuchal region of Heloderma are very much blended together, and consequently difficult of dissection and individualization. Hoffmann has also called this muscle the spinalis dorsi, but incorrectly quotes Sanders as having termed it the " sphincter dorsi" (Bronn's ' Thier-Reichs,' Bd. vi. Abth. iii. p. 618, quoting P. Z. S. 1872, p. 161). 57- The Longissimus dorsi may almost be considered as the direct extension forwards of the supracaudal muscles, for it is only at the leading sacral vertebra, superficially, that we can detect a semi-distinct, transverse, line of demarcation that seems to indicate the point where a blending takes place among the caudal muscles on the one hand, and the longissimus dorsi and the sacro-lumbalis on the other. Along the dorsum the present muscle is quite intimately united, by an intervening bond of semideuse fasciae, with |
