THE SKELETON OF THE VERTEBRATE HEAD. 107 



bridge-like connections of neighbouring sclerotomes are not 

 unfrequently completed by the fibrous basis of the sclerome. 

 In birds, and the typical lacertians, indeed, in which the 

 actinapophyseal elements exhibit remarkable adaptations, the 

 fibrous matrix in which they are imbedded, and by which 

 they are connected, forms an essential feature of their arrange- 

 ment. 



The actinapophyseal elements of a sclerotome are to be 

 distinguished as hsemal and neural those attached to the 

 haemal and those connected with the neural arches. The 

 hsemactinapophyses are the most usual and numerous, and 

 have hitherto been alone recognised as such by anatomists. I 

 shall therefore at present only remark, in reference to the 

 neuractinapophyses, that I consider as such the neural range 

 of " additional ribs," the interspinous bones and rays of the 

 dorsal fins, and of the neural half of the caudal fin in cartila- 

 ginous fishes, and also the inter-neural cartilages, to which 

 attention was first directed by Joh. Muller. In the cephalic 

 sclerotomes, the neuractinapophyses constitute the so-called 

 "sense capsules" and the system of " muco-dermal bones." 

 The so-called " muco-dermal bones" have been latterly referred 

 by the continental anatomists to the dermo-skeleton. I am, 

 nevertheless, inclined to believe, that when the general mor- 

 phological relations of these bones, and their existence in at 

 least reptiles and birds, are taken into consideration, they will 

 be admitted as elements of the endo-skeleton. They are not 

 the only bones in the head of the osseous fish which are tra- 

 versed by mucous tubes ; but from their superficial position 

 they generally are so, and from the same circumstance are 

 frequently overlaid by dermal bone. Professor Owen has 

 adopted the doctrine of the muco-dermal character of these 

 bones, and includes the lachrymal among them. Believing 

 the lachrymal to be a cephalic neuractinapophysis, I cannot 

 assent to the rejection of this bone from the endo-skeleton, 



