The Cranial System

As has been stated, no protovertebrae appear on the floor of the skull; nor is there any cranium, nor any developmental stage of a cranium, in which separate cartilaginous centres are known to occur in this region.

On the contrary, when chondrification takes place, it extends continuously forward, on each side of the notochord, 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 trabeculae cranii. In front of the fossa, the trabeculse reunite and end in a broad plate, usually bifurcated in the middle line-the ethmovomerine 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 may be developed in the more anterior parts of the skull do not ordinarily reach its roof, but leave a wide, 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 ethmovomerine part of the skul;; while, in that of the eyes, it appertains 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 distinct or not, it early forms one mass with the skull, immediately 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 partition between them. The partition is the proper ethmoid, the lamina perpendicularis of human anatomy; the posterolateral 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 acquires a larger surface, are the turbinals.

Riblike cartilaginous rods appear in the first, second, and, more or fewer, of the succeeding, visceral arches in all but the lowest Vertebrata. The upper ends of the first and second of these become connected with the auditory capsule, which lies immediately above them.

The first visceral arch bounds the cavity of the mouth behind, and marks the position of the mandible or lower jaw. The cartilage which it contains is termed Meckels cartilage.

The cartilaginous rod contained in the second visceral arch of each side is the rudiment of the hyoidean apparatus. Like the preceding, it unites with its fellow in the ventral median line, where the so-called "body" of t he hyoid arises.

A ridge, continued forward from the first visceral arch to the olfactory sac (Fig. 4, F; Fig. 8, l), bounds the mouth on each side, and is called the maxillary process. A cartilaginous palato-ptetygoid rod, developed in this process, becomes connected with Meckel's cartilage behind, and with the prefirontal cartilage in front.

The maxillary process is at first separated by a notch corresponding with each nasal sac, from the boundary of the antero-median part of the mouth, which is formed by the free posterior edge of a, fronto-nasal process (Fig. 4, F; Fig. 8, k). This separates the nasal sacs, and contains the cartilaginous, ethmovomerine, anterior termination of the skull. The notch is eventually obliterated by the union of the fronto-nasal and maxillary processes, externally; but it may remain open internally, and then gives rise to the posterior nasal aperture, by which the nasal cavity is placed in communication with that of the mouth.