OF BAL^XOPTERA MUSCULUS 



219 



Foramina. — The nutritive foramina of the atlas have been noticed. The principal seats 

 of these foramina in the majority of vertebrae are towards the ends of the body, near the 

 root of the transverse processes, and on the upper and lower surfaces of the body near 

 the middle. In the dorsal region the superior foramina are usually two, symmetrically 

 placed side by side, and this arrangement continues with occasional variations. On the 

 lower surface a similar arrangement begins to prevail towards the posterior end of the lum- 

 bar region. In the 7th caudal, the anterior edge of the transverse process presents a deep 

 notch ; in the 8tb its base is broad and perforated by a foramen which continues until all 

 trace of a transverse process is lost in the 14th, after that, one or more foramina in a cor- 

 responding position are visible entering the bone, till the last two or three are reached. In 

 the 13th and 14th caudal, the projections at the ends of the vertebra for the chevron 

 bones become united into ridges with a central perforation. Subsequently two proportion- 

 ally large foramina appeal' side by side, entering the bone, and persist till the last three. 

 It is very probable, reasoning from analogy, that the perforations of the transverse pro- 

 cesses are for vessels, because after these processes have ceased to exist I have found that 

 a wire passed into one of the inferior foramina emerges through the lateral one and if it 

 be then thrust in again can l)e brought out above. The inferior foramina continue close 

 together, those above become more or less separated ; but the communication -n-ith slight 

 variations continues the same. 



The Spinal Canal. — The vertical and transverse diameters of this canal are about equal 

 in the first two vertebra? ; in the remaining cervical the transverse greatly exceeds the other. 

 In the 2d and 3d dorsal the height of 

 the canal increases considerably, and in 

 the 5th, the two diameters are about 

 equal. Shortly afterwards the vertical 

 prevails and the next change is the 

 shortening of the transverse. The last 

 dorsal presents a canal having the form 

 of an isosceles triangle, which continues 

 about the same through the lumbar re- 

 gion. Towards the beginning of the 

 caudal region it becomes smaller, in 

 the 15th it is barely pervious, and in the 16th does not exist. 



From this description and a study of the table, the shape of any given vertebra may be 

 pretty accurately deduced, but some further account of the latter part of the caudal region 

 may be convenient. The 15th caudal is a transition from the vertebrae bearing processes 

 to those bearing none. It is the last showing any sign of a spinal canal, which here is so 

 small that one end will not admit the little finger. It is covered by a bridge of bone which 

 shows no signs of a spinous process or of metapophyses. The transverse processes had dis- 

 appeared in a previous vertebra. The inferior processes for the chevron bones are two mere 

 swellings without the perforation shown in the 13th and 14th. The 16th is about round, 

 without any striking characteristic. The 17th looks rather square and the succeeding ones 

 are decidedly so ; they have two foramina close together, on the lower side and two above 

 at some distance apart. The 24th and 25th show a want of regularity and of anything 

 characteristic. The 26th (PL vi, fig. 8), the last, tapers towards its posterior end, which 



FijT. 10.— 6th Caudal Vertebra. 



