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Show 1885.] OF THE HUMAN SPHENOID BONE. 579 After the trabeculae have coalesced, growth occurs in them in three directions:- 1. Laterally, to form the side walls of the skull anterior to the penotic capsules. 2. Mesially, to fill up the floor of the pituitary fossa. 3. Forwards, to form the ethmo-vomerine and fronto-nasal plates. m Although the trabecule at a very early period form a floor to the pituitary fossa, yet this fossa is never completely shut off from the pharynx in the chondro-cranium ; a small opening persists for a very long time, the consideration of which leads to some very interesting conclusions, and which renders necessary the study of the early stages of the formation of the mouth and pharynx. It has been very satisfactorily proved that the buccal mucous membrane is derived from the epiblast; the process by which this derivation occurs is usually described as a tucking-in of the epiblast ; but. in reality it is a necessary outcome of the primary cranial flexure Passing between the open arms of the trabeculae is a narrow tubular portion of the anterior primary encephalic vesicle, known as the infundibulum ; this diverticulum from the primitive brain comes into contact with the buccal epiblast; the meeting point of the two structures is represented by the pituitary body. This disposition of the parts has long been known. Whilst engaged working over the development of this complex region, I found that even at the mid period of intra-uterine life of the human embryo a narrow cavity may be detected passing from the pharynx through the basisphenoid, so as to come into close relation with the infundibulum. The point of communication with the pharynx is in the middle line in contact with the basisphenoid, the spot being indicated at birth by a recess in the mucous membrane known as the bursa pha-ryngea (see fig. 2, p. 580). After birth the canal suffers obliteration ; but a band of fibrous tissue, passing from the pituitary body to the pharynx, represents the original position of the canal. When the bone is macerated the fibrous tissue disappears, leaving a hole in the basisphenoid, termed by Landzert the canalis cranio-pharyngeus (Petersburger med. Zeitschrift, Bd. xiv.). This cranio-pharyngeal canal may be detected in the floor of the sella turcica in very many mammals at birth. Up to the present time I have been unable to assure myself that the lower end of the infundibulum ever opens into the pharynx, but there is every probability that such is the case. However, the existence of this diverticulum of the first encephalic vesicle raises an exceedingly interesting question. It will be remembered that the central canal of the spinal cord at its caudal end is brought into relation in the early embryo of very many of the Vertebrata with the hind gut, by a narrow and, in most cases, very temporary passage known as the neurenteric canal, which passes around the caudal end of the notochord, but afterwards becomes obliterated. Turning now to the cephalic end of the notochord, we find the |
