426 



NATURE 



{April 2, 1874 



branch (VI 1=) going to the inner (posterior) side of the 

 mandiLular arch, and its posterior (VI If) division to the 

 outer side of the hyoid. The glossopharyngeal (IX.), in 

 like manner, is distributed to the inner side of the hyoid 

 and the front face of the first branchial, the hinder 

 face of which, as well as all the remaining branchial 

 arches, is supplied by the great tenth nerve, or vagus (X). 

 By Rathke, to whom are due the first accurate obser- 

 vations on thi development of the skull, the trabecular 

 arches were looked upon as mere forward processes of the 

 investing mass, and were called ti-abt-cula: cranii, or 

 " rafters of the skull." This misconception of their true 

 nature arose from the fact that they very soon coalesce 

 with the investing mass, and are only to be found distinct 

 in extremely early stages. 



c. Round the organs of smell and hearing cartilaginous 

 investments are formed, known respectively as the nasal 

 (Na) and auditory (Au) cap>:ules. The latter become, at 

 a very early period, united with the investing mass, while 

 the nasal capsules come into close relations with the ante- 

 rior or distal ends of the trabecular.* These are t)\e para- 

 7!C!nal eU-incnts of the primordial skull. 



d. Certain cartilages may be developed in relation with 

 and external to the visceral arches, called from this cir- 

 cumstance " extra-viscerals." Of this nature are the 

 labial cartilages, which take so large a share in the forma- 

 tion of the skull of many cartilaginous fishes. 



c. Lastly, the general membranous roof and walls of 

 the brain-case may chondrify to a greater or less extent, 

 but this chondrification is in nearly every case continuous 

 with the trabecular and nasal capsules in front and below, 

 and with the investing mass and auditory capsules be- 

 hind. 



Not only is the originally membranous cranium thus 

 strengthened by deposits of cartilage, but osseous 

 deposits may take place either in the primordial skull 

 itself, or in the subcutaneous tissue surrounding it. 

 The latter are called " investing-boncs," or paivsiost's : 

 the former may be of two kinds ; when occurring as mere 

 calcifications of the substance of the cartilage, they are 

 known as c?idostoscs, when having the structure of true 

 bone, as ectostoscs. 



\\.— Skull of the Shark {Lesser Spotted Doi;-fish, Scyl- 

 lium canicula). — The skull of the shark is one of the best 

 examples of the chondro-cranium in its least altered state, 

 being entirely uncomplicated by the development of in- 

 vesting bones, and covered simply by a close-set series of 

 superficial calcifications. 



The brain-case is much flattened both above and below. 

 Seen from above, it is greatly excavated in its central 

 portion by the orbits, but expanded in front by the 

 rounded nasal capsules (Fig. 2, Na), and behind by the 

 more or less quadrate auditory capsules (Au). The carti- 

 laginous roof of the skull, or iegintn eranii, is interrupted 

 by an oval membranous space, or " fontanelle," situated 

 between the hinder boundaries of the nasal sacs, a posi- 

 tion peculiar to the sharks. The upper surface of the 

 otic capsules exhibits three well-marked elevations for the 

 semi-circular canals, and just within that for the an- 

 terior canal, a small rounded aperture, the remains of 

 the primitive involution of the integument from which the 

 organ of hearing arose. An elevation on the hinder end 

 of the posterior canal marks the position of the epiotic 

 ossification so well developed in the osseous fish ; the 

 pterotic is also indicated by a large outstanding process 

 (Pt O) which forms the postero-external angle of the 

 skull, and the sphenotic (post-frontal of Cuvier) by the 

 posterior portion of the supra-orbital ridge (S Or) when it 

 coalesces with the auditory capsule (Sp O). The anterior 



* The Sclerotic, the fibrous (mammalia), or cartilaginous (Sauropsida and 

 Ichthyopsida) capsule developed around the organ of sight (E) never really 

 forms part of the sliull, although in the sharks and rays, and some osseous 

 fish, it is articulated with the side walls of the brain case by a cartilaginous 

 pedicle. The form of the skull is, however, greatly governed by the presence 

 ot these optic capsules. 



extremity of this ridge forms in like manner the pre-frontal 

 process. One very noteworthy point, observable both in 

 an upper, under, or side view, is the presence between the 

 nasal capsules of a short rod of cartilage (B Tr) repre- 

 senting the median basal portion, or keystone of the tra- 

 becular arch, and hence called the basi-trabccular. 



Viewed from behind, the skull presents a large foramen 

 magnum, bounded below and at the sides by the well- 

 developed occipital condyles, between which is a slight 

 elevation, showing the point where the notochord origi- 

 nally entered the investing mass. External to the occi- 

 pital forainen, and marking the original boundary between 

 the parachordal and otic eleinents of the skull is the fora- 

 men for the exit of the 9th and loth nerves. The tri- 

 geminal foramen, which always points to the anterior 

 limit of the otic region, forms a large aperture in the side 

 wall of the brain- case (V), as also does the optic foramen 

 (II). 



The jaws are very loosely united to the other parts of 

 the skull, and consist of an upper and a lower dentigerous 

 arch, the former of which is connected with the skull by 

 two bands of ligamentous fibres. The lower arch (Mck), 

 which articulates with the posterior end of the upper, is 

 the homologuc of Meckel's cartilage, the rod which forms 

 the foundation of the lower jaw in all vertebrata, but 

 which as a rule, owing to the great development of invest- 

 ing bases, is reduced to a more slender style, or is even 

 suppressed altogether. The posterior portion of the 

 upper dentigerous arch (Ou) answers to the quadrate, a 

 bone which in all Teleostei, as well as in Amphibia and 

 Sauropsida, gives attachment to the mandible. The re- 

 mainder of this " upper-jaw " represents the scries of bones 

 (pterygoid, meso-pterygoid, and palatine) which in the 

 osseous fish connect the quadrate with the fore-part of the 

 skull, the meta-pterygoid or proximal end of the mandi- 

 bular arch being represented by the band of fibrous tissue 

 (M Pt) which connects the quadrate with the auditory 

 capsule. 



Close behind the attachment of the meta-pterygoid 

 ligament, a large phalangiform cartilage (H M) is articu- 

 lated to the auditory capsule ; this represents the hyo- 

 mandibular of the osseous fish, the largest bone in the 

 suspensory apparatus of the lower jaw, and the uppermost 

 portion of the hyoid arch. Between this cartilage and the 

 meta-pterygoid is a space (Sp), which in the recent state 

 forms a communication between the cavity of the mouth 

 and the exterior. This is called the spiracle, and answers 

 to the tympano-Eustachian passage of the hi;;her verte- 

 brata. The distal portion of the hyoid arch consists of a 

 large and strong cartilage, the cerato-hyal (C Hy), below 

 which is a basal piece, common to both sides (shown in 

 the figure by dotted lines), the basi-hyal. This is an ex- 

 tremely simple mode of segmentation of the hyoid arch, 

 and approaches nearly to the primitive condition. 



The cerato-hyal is connected with the mandible by a 

 ligament — the mandibulo-hyoid ligament (mhl). 



There are five branchial arches, all of which are split up 

 into four segments, called, after the names originally 

 given by Prof. Owen to the corresponding parts in the 

 Teleostei, pharyngo- epi-,cerato-,and hypo-branchial. The 

 inferior median piece, or basi-branchial (B Br), occurs 

 only in the hinder part of the series. 



The extra-viscerals are represented by the labial carti- 

 lages (1" — 1') and by the extra-branchials (Ex Br i — 5), 

 between which and the branchial arches extend carti- 

 laginous rods, acting as supports to the septa between the 

 gill-pouches. The last arch, however, bears no gill and 

 has no extra-branchial conesponding to it. The hyoid 

 also is devoid ol an extra-visceral, although it bears a series 

 of greatly divided cartilages, which support the anterior 

 wall of the first gill-sac ; this arch, consequently, carries 

 a half-gill. The branchia of the mandibular arch is repre- 

 sented by a vascular plexus (pscudo-branchia) on the 

 anterior side of the spiracular opening. 



