THE SKELETON OF THE VERTEBRATE HEAD. 99 



The processes which complete the canal under the posterior 

 cranial and anterior trunk centrums in certain fishes, and of 

 the cervical centrums in certain birds, are probably of the 

 same nature as the chevron-bones, which, according to Joh. 

 Miiller, appear to be developments of the inferior pair of con- 

 stituent pieces of the centrum. 



We are entitled, then, to require that every part to which 

 the pleurapophyseal or haemapophyseal character is attributed, 

 should have been proved, by direct observation or otherwise, 

 to have been developed in the " ventral folds." 



It appears to me very doubtful whether there are suffi- 

 cient grounds for limiting the number of morphological 

 elements in the haemal arch to one pair of " hsemapophyses " 

 and a "haemal spine ;" or to a pair of " pleurapophyses," a 

 pair of "hsemapophyses," and a "haemal spine;" while an 

 increase in the number of sclerous pieces is accounted for by 

 the principle of "vegetative repetition," or " teleologically." 

 While I admit the grouping of the elements of the more corn- 

 neuroses, and, embedded in it, the haemal divisions of the spinal nerves extended 

 outwards, downwards, and backwards, like a series of intercostal nerves. The 

 lateral muscular mass of the tail arranged in myotomes with metamyotomic 

 fibrous laminae, nearly as distinct as in the fish, lay on the outside of the layer 

 of fat. Each of the lateral aponeurotic cavities was occupied by the " femoro- 

 peroneo-coccygien " muscle of Cuvier, which arose from the under surfaces of 

 the transverse processes, the sides of the chevron bones and mesial aponeurosis, 

 and passed out of the cavity through a space left in its outer wall behind the 

 ischium to be inserted into the thigh-bone. The mesial membrane divided 

 above, its two laminae corresponding to the limbs of the chevron -bones, and 

 passing in front into the walls of the pelvis. 



This arrangement appeared to me to indicate that the transverse processes, 

 the lateral aponeuroses, and the haemal divisions of the spinal nerves, were in 

 the position of the proper haemal arches of the tail ; that the two aponeurotic 

 chambers constituted in fact, together, the abdominal or visceral cavity, 

 divided by the mesial lamina, and occupied by a pair of muscles, referable to 

 that group of muscles which in the trunk lie on the inner surface of the vis- 

 ceral chamber, and that therefore the chevron-bones are not real haemal arches, 

 but subcentral developments. 



