22 THE ANATOMY OF YERTEBRATED ANIMALS. 



and usually invests the anterior termination of that body, 

 more or less completely, as a basilar plate. 



The basilar plate does not extend under the floor of the 

 pituitary fossa, but the cartilage is continued forward on each 

 side of this, in the form of two bars, the traheculm cranii. In 

 front of the fossa, the trabeculae reunite and end in a broad 

 plate, usually bifurcated in the middle line — the ethmovome- 

 rine plate. 



On each side of the posterior boundary of the skull, the 

 basilar cartilage grows upward, and meets with its fellow in 

 the middle line, thus circumscribing the occipital foramen^ 

 and furnishing the only cartilaginous part of the roof of the 

 skull; for any cartilaginous upgrowths which maybe devel- 

 oped in the more anterior parts of the skull do not ordinarily 

 reach its roof, but leave a "svide, merely membranous space, or 

 fontanelle^ over the greater part of the brain. 



Before the skull has attained this condition, the organs of 

 the three higher senses have made their appearance in pairs 

 at its sides ; the olfactory being most anterior, the ocular 

 next, the auditory posterior (Fig. 4). 



Each of these organs is, primitively, an involution, or sac, 

 of the integument ; and each acquires a particular skeleton, 

 which, in the case of the nose, is furnished by the ethmovo- 

 merine part of the skull ; while, in that of the eyes, it apper- 

 tains to the organ, is fibrous, cartilaginous, or osseous, and 

 remains distinct from the skull. In the case of the ear, it is 

 cartilaginous, and eventually osseous : whether primitively dis- 

 tinct or not, it early forms one mass with the skull, immedi- 

 ately in front of the occipital arch, and often constitutes a very 

 important part of the walls of the fully-formed cranium. 



The ethmovomerine cartilages spread over the nasal sacs, 

 roof them in, cover them externally, and send down a parti- 

 tion between them. The partition is the proper ethmoid, the 

 lamina perpendicularis of human anatomy; the posterolat- 

 eral parts of the ethmovomerine cartilages, on each side of the 

 partition, occupy the situation of the prefrontals, or lateral 

 masses of the ethmoid of human anatomy. The ingrowths of 

 the lateral walls, by which the nasal mucous membrane ac- 

 quires a larger surface, are the turhinals. 



Ribhke cartilaginous rods appear in the first, second, and, 

 more or fewer, of the succeeding, visceral arches in all but the 

 lowest Vertehrata. The upper ends of the first and second of 

 these become connected with the auditory capsule, which Ues 

 immediately above them. 



