Comparative morphology of the semispinalis-spinalis muscle of snakes and correlations with locomotion and constriction.

Segments of the spinalis portion of the M. semispinalis-spinalis (SSP) were examined in 107 snakes representing 94 species, 85 genera, and 11 families. Allowing for slight variation within individuals and species, the following generalizations can be made. (1) Three major types of segments of the SSP were found in Typhlops, booids, and colubroids. (2) Within each type, differences in the segmental length of the spinalis result primarily from different lengths of the anterior tendons. (3) Specializations in habitat and locomotor modes usually account for variations in the segmental lengths of the spinalis. (4) Constrictors seem to have undergone selection for increased flexibility which is gained by having relatively more vertebrae and, in some cases, shorter muscle segments.