256 SKULL 



the^adult skull. As these will be fully dealt with in the volume on Osteology, it 

 will suffice here if a brief account be given of the chondrocranium as a basis for 

 that description. 



The cartilaginous cranium consists of two parts, the neurocranium and the 

 visceral skeleton. The neurocranium consists of two sections, a parachordal and a 

 prechordaL''-*; The auditory capsule becomes an integral part of the parachordal, and 

 the olfactory capsule of the prechordal section. The basicranial plate encloses 

 the notochord, which ends at the dorsum sellee in a rounded process, its anterior 

 hooked portion having disappeared. According to Froriep, Robinson, and others, ' 

 the chorda lies on the ventral aspect of the middle portion of the basicranial plate, 

 being completely invested by cartilage only behind and in the dorsum sellae. The 

 plate consists, as mentioned above, of an otic and an occipital segment. From the 

 latter there extends on each side a lateral plate (occipital pillar, Gaup), which in 

 man is not vertical, as in lower mammals, but is laid out horizontally. This becomes 

 continuous with the posterior edge of the auditory capsule behind a gap which is 

 left as the jugular foramen. In each lateral plate which represents the fused occipital 

 arches is the foramen for the hypoglossal nerve. At first the skull is wide open 

 behind the hind-brain, being covered merely by the membrana reunions ; but into 

 this, from the auditory capsule and occipital pillars on each side, a plate of cartilage 

 extends to close in the foramen magnum and form the supra-occipital. The anterior 

 part of the basicranial plate becomes continuous with the auditory capsule. This 

 is an oval mass of cartilage with its long axis directed inwards and forwards, and 

 divided into an upper and posterior vestibular portion, and a lower and anterior 

 cochlear portion. Between these is a groove on the upper aspect for the facial nerve, 

 which in the figure is seen partially bridged over to form a canal. On the mesial 

 aspect of the capsule is seen the internal auditory meatus, and a deepish fossa 

 (fossa subarcuata) which extends below the superior semicircular canal. On the 

 outer aspect a shelf-like process extends which ultimately forms the tegmen tympani. 

 The prechordal or trabecular portion of the skull stands at first at a considerable 

 angle from the posterior portion (Hagen). There are, as already mentioned, no 

 separate trabecular cartilages formed in man. Chondrification occurs in an 

 already continuous mesial blastema, which is interrupted by the stalk of the 

 pituitary body. The cartilage extends directly forwards into the mesial nasal 

 process as the nasal septum. On the upper aspect of the cartilage in front of the 

 dorsum sellae is a shallow fossa the future sella turcica, the floor of which is 

 incomplete for a time where the stalk of the pituitary body passes through it. 

 A process projects from the cartilage on either side of the fossa ; between this and 

 the auditory capsule the internal carotid artery enters the skull. To this process 

 is attached an obliquely placed plate of cartilage (ala temporalis), arising from a 

 separate centre (Levi), which is the rudiment of the great wing of the sphenoid. 

 As it extends backwards it surrounds the mandibular division of the trigeminal 

 nerve and the middle meningeal artery so as to form the foramen ovale and foramen 

 spinosum. Between it and the orbital wing is a wide gap through which the 

 maxillary division of the fifth and all the nerves entering the orbit pass. By the 

 formation of a bridge of cartilage the maxillary division is separated from the 

 rest of the nerves ; thus the foramen rotundum is formed, while the main fissure 

 becomes the sphenoidal fissure. In front of the sella turcica a broad bar of 

 cartilage expands laterally on each side into the orbital or lesser wings (ala orbitalis). 

 From the posterior borders of each orbital wing a bar is developed which is 

 attached to the side of the central cartilage in front of the pituitary fossa and 

 completes the optic foramen. From the anterior border of each orbital wing a 



1 See Robinson, Jour, of Anat. and Phys., xxxviii. 



