126 ANATOMY OF THE WOOD EAT 



slightly better so. The third vertebra has a width of about 

 10 mm., the seventh 13 or 14, and they average about 6.5 

 mm. in height. 



The other muscles that are with certainty attached to the 

 five posterior, cervical vertebrae are the scalenus, longus 

 capitis, longus colli, longus atlantis, and levator scapulae 

 upon transverse processes, and complexus, spinalis dorsi, and 

 transversospinales to other portions. In addition, other 

 muscles which are considered as arising from the middorsal 

 line most certainly influence the length of the spines of all 

 the vertebrae concerned. 



Thoracic vertebrae 



There are thirteen pairs of ribs in the genus Neotoma 

 arising from the same number of thoracic vertebrae. In 

 the adults available the visible portion of the thoracic series, 

 measured upon the dorsum between the spines, is 51.5, 50, 

 and 45.4 mm. in Homodontomys, or 35.3, 36.2, and 34.5 

 per cent of the vertebral body length. In Neotoma the 

 figures are 40, 40, and 41 mm., or 36.7, 38, and 38.8 per cent 

 of the body length. In Teonoma they are 55 and 51 mm., 

 or 37.4 and 37.2 per cent of the body length. (In N. p. 

 pennsylvanica this percentage is 40.) 



The dorsal aspect of the first thoracic vertebra is typically 

 similar in the majority of cases to the characteristic form of 

 the cervical vertebra, but in a single specimen of Homodon- 

 tomys the spine is about 3 mm. in length — as long as that of 

 the fourth. The spine of the second vertebra is much 

 longer than any of the others, although this character 

 varies both individually and with age. It may reach a 

 length of over 8 nam., or be but half that height. In Homo- 

 dontomys and Neotoma the spines of the remaining thoracics 

 are practically uniform in height, measuring three or four 

 millimeters; but in Teonoma the third and fourth are longer, 

 being graduated in decreasing size caudad from the second 



