245

LECTURE XIII.

ON THE STRUCTURE OF THE SKULL,

THE SKULLS OF MAMMALIA.

WE have met with no important difficulties in the way of identifying the bones of the Bird's skull with those found in the skulls of the Reptilia and still lower Vertebrata; and hence, if the cranium of a Mammal be compared with that of a Bird, the bones which correspond in the two will obviously be homologous throughout the series.

The accompanying figure represents a longitudinal and

Fig. 97.-Longitudinal and vertical section of the cranial cavity of a Beaver.

vertical section of the skull of a Beaver (Castor fiber), drawn of the same absolute length as the section of the Ostrich's skull

(Fig. 88, A), and exhibiting a corresponding extent of the cranium. The three segments of the basi-cranial axis are at once recognisable and identifiable with the basi-occipital, basi-sphenoid, and presphenoid of the Bird; but the basi-sphenoid is truncated at its anterior end, as in the Crocodile, not produced into a long beak, as in Birds and many Lizards. The ex-occipital and supra-occipital bones, again, have all the relations of those of the Ostrich, and are universally admitted to be the homologues of the latter.

In the Ostrich, as we have seen (Fig. 88, A), there lies in front of the ex-occipitals a large bony mass, composed of the confluent opisthotic, epiotic, and pro-otic bones. The inner face of the single periotic bone thus formed is divided into two surfaces, one anterior and one posterior, by a ridge which runs somewhat obliquely from above downwards and forwards. The anterior surface is concave, looks somewhat forwards, articulates in front with the alisphenoid, and contains no part of the organ of hearing; the third division of the trigeminal nerve passes out in front of it. The posterior surface presents, inferiorly, the apertures for the portio dura and the portio mollis; superiorly and in front, a fossa arched over by the anterior vertical semicircular canal; while, superiorly and behind, it contains the posterior vertical semicircular canal. Between the posterior edge of this division of the bone and the ex-occipital the eighth pair of nerves leaves the skull.

In the Beaver (Fig. 97), there is a single mass of bone not dissimilar in form and proportional size, which has always been admitted to be the homologue of the pars petrosa and pars mastoidea of Man, and the general relations and characters of which may be described in exactly the same terms. The inner face is divided into two surfaces, one anterior and one posterior, by a ridge which runs, somewhat obliquely, from above downwards and forwards. The anterior surface is concave, and looks slightly forwards; it articulates in front with a bone which, as all agree, corresponds with the alisphenoid of Man, and lies behind the exit of the third division of the trigeminal. The posterior division presents, inferiorly, the apertures for the portio dura and portio mollis; superiorly and in front, a fossa arched over by the anterior vertical semicircular canal; while superiorly and behind

it contains the posterior vertical semicircular canal. Between the hinder edge of this division and the ex-occipital, the eighth pair of nerves leaves the skull.

The inferior, or internal, edge of the periotic bone in the Bird, and that of the pars petrosa in the Beaver, comes into relation with the basi-occipital and basi-sphenoid; externally, each exhibits the fenestra ovalis and rotunda, and is related, above, to the squamosal.

In fact, the only noteworthy differences between the ornithic periotic, and the Mammalian pars petrosa and mastoidea, are that the former becomes anchylosed with the adjacent bones of the cranial wall, while the latter remain separate from them; and that, while the periotic articulates, above, with the parietal in the Bird, the corresponding ossification in the Mammal is separated from that bone by the squamosal.

On the former distinction it would of course be absurd to lay any weight; as regards the latter, it is deprived of all significance by the circumstances that in some Birds-as, e.g., the common Fowl-the squamosal interposes between the periotic and the parietal in the wall of the skull; and that in some Mammals-as, e.g., the Sheep-the squamosal is completely excluded from the skull, and the pars petrosa unites with the parietal.

The simple anatomical comparison of the parts appears, then, to be amply sufficient to demonstrate, that the pars petrosa and mastoidea of the Beaver correspond in every essential respect with the periotic mass of the Bird, and therefore with the pro-otic, opisthotic, and epiotic bones of Reptiles and Fishes. On the other hand, no one has ever doubted that the petrosal and mastoid of the Beaver answer to the petrosal and mastoid of Man; and therefore we are led by the comparison of adult structure, merely, to exactly the same conclusion as that at which we arrived by the study of development, to wit, that the pars petrosa and pars mastoidea of Man answer to the periotic bones of the lower Vertebrates.

In front of the periotic, the side wall of the cranium is formed by an alisphenoid, anchylosed below with the basi-sphenoid; and, still more anteriorly, by a large orbito-sphenoid, united inferiorly with the presphenoid, which is distinct from the basi-sphenoid behind, and from the ethmoid in front.

In the roof of the skull (Fig. 98) a large inter-parietal, SO1, which corresponds with the upper part of the squama occipitis of Man, occupies an interval left, posteriorly, between the two

Fig. 98. Side view of the skull of a Beaver. Ty, tympanic bone; M, pars mastoidea; Pm, the downward process of the ex-occipital, called " paramastoid."

parietals; otherwise, the bones correspond with those found in the roof of the skull of the Bird. The ethmoid, the vomer, the nasal bones, the premaxillæ, maxillæ, lachrymal, jugal, and squamosal bones, the palatines, and the pterygoids of the Rodent, present no difficulties to the student acquainted with the structure of the Bird's and Reptile's skull; but he will miss the pre-frontals, the post-frontals, the quadrato-jugal, the transverse, and the quadrate bones, together with all the pieces of the lower jaw, save the dentary.

The post-frontals, the quadrato-jugal, the transverse, and four out of the five missing pieces of each ramus of the lower jaw, appear to have no representatives in the Mammalian skull.

The pre-frontals, on the other hand, are represented by the so-called "lateral masses of the ethmoid," with their developments, the superior and middle ethmoidal turbinal bones, which answer precisely to those of Man. A third turbinal, developed from the primitive cartilaginous wall of the olfactory chamber, eventually becomes united with the maxilla, and answers to the inferior or maxillary turbinal of Man; while, in the Beaver, there is a fourth turbinal, connected with the superior turbinal and with the nasal bones, which may be termed the "nasal turbinal."

How far these well-defined turbinals of the Mammal answer to the cartilaginous and osseous turbinals of Birds and Reptiles, is a question which has yet to be elucidated.

I have already endeavoured to show that the quadrate and articular bones of the oviparous Vertebrata are represented by the incus and malleus of Man, and consequently by the corresponding bones in all Mammalia; and that, as a consequence of the appropriation of two bones of the mandibular series to the uses of the organ of hearing, the dentary bone develops its own condyle, and articulates with the squamosal.

Another bone which appears to have no distinct representative in most oviparous Vertebrates is very conspicuous in most. Mammals, and far more so in the Beaver than in Man. This is the tympanic element, and it will be useful to study with especial attention the characters of this bone, its relations to the periotic, and the manner in which both are connected with the other bones of the skull.

Fig. 99.

In a transverse section of the conjoined tympanic and periotic bones, taken through the canal which is common to the anterior and posterior vertical semicircular canals (Fig. 99), the periotic mass is seen to be prolonged, external to and below the horizontal semicircular canal and that for the passage of the portio dura, into a stout "mastoid process" (M), which appears upon the outer surface of the skull, between the ex-occipital, the squamosal, and the tympanic, as a production downwards and outwards of the "pars mastoidea," which is doubtless, as in Man, composed partly of the pro-otic and partly of the epiotic and opisthotic bones.

Fig. 99.-A vertical section of the conjoined tympanic and periotic bones of the Beaver (Castor fiber). a, external auditory meatus; b, groove for the tympanic mem. brane; c, the inner lip of the tympanic; Eu, Eustachian tube; Cl, cochlea; M, pars mastoidea.

The tympanic bone is produced, externally, into a spout-like tube, directed forwards and upwards, which is the external auditory meatus (Au, Fig. 98); below and internally the tube dilates into a thin walled hemispherical bulla (b, c, Fig. 99), open superiorly, and produced in front and anteriorly into a perforated process, which contains the osseous part of the Eustachian tube.

*I learn from Mr. Parker that all Birds above the Struthionida have a more or less perfect chain of tympanic bones, of which there are six in Corvus corone.