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 vertebrcC 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 tiiil 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 tlie vertebrse, 

 are each in turn inserted into a neural spine of one of the same. 

 Still more deejily 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 them- 

 selves. All these I take to be tendons of insertion of the spinalis 

 dorsi, and cutting down more deeply on the muscle we find its oiigiu 

 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 Ugamentum 

 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, and to some 

 slight 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 

 tij) 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 siipracaudal upon either side ot 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. Hoifmann 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). 



.')". 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-lumljalis 

 on the other. Along the dorsum the present muscle is quite 

 intimately united, by an intervening bond of semideuse fasciae, with 



