THE SKELETON OF THE VERTEBRATE HEAD. 115 



series of ganglionic masses at the base of the brain which are 

 serially homologous with the spinal cord. 



EHINAL SCLEROTOME. In Mammals. The principal parts 

 of the cranium which remain unossified in the mammal are 

 the nasal septum and the cartilages of the nose. Of these, the 

 unossified portion of the nasal septum is the anterior prolon- 

 gation of the basal portion of the so-called " primordial 

 cranium." It is consequently a continuous mass of cartilage, 

 but is nevertheless referable to three sclerotomes ; its superior 

 portion completing the centrum of the ethmoidal ; its lower 

 portion the centrum of the vomerine ; and its anterior that 

 of the rhinal sclerotome. 



The rhinal sclerotome in the mammal is fibrous, cartila- 

 ginous, and catacentric. Its centrum, formed by the anterior 

 portion of the nasal septum, extends from its neural to its 

 haemal margin. Its right and left neural elements are the so- 

 called superior or triangular cartilages of the nose. They 

 may be continuous with or merely attached to the neural edge 

 of their centrum. The anterior margins, or angles of these 

 cartilages, and the corresponding point of the septum or cen- 

 trum is the absolute anterior termination of the animal, or 

 more precisely of its morphological axis. The ridge of the 

 nose downwards and forwards to that point is neural or dorsal ; 

 beyond it, although continued in the same line, the ridge is 

 haemal or sternal. 



The two alar cartilages are the haemapophyses of this 

 sclerotome. Variously modified in form, they are more or less 

 firmly attached to the lower margins of the upper cartilages. 

 In front they are attached to the septum, to which also they 

 are more loosely connected round the tip of the nose, being 

 frequently folded in on the ridges of the septum. In the 

 fibrous membrane occupying the sides of the space between 

 the posterior margins of the alar cartilages which together 

 constitute the haemal arch of their sclerotome, and the ante- 



