CRANIUM. 



725 



less the same; and a glance at the seveval 

 classes of vertebrated animals will demonstrate 

 that security for the brain is the grand aim of 

 the contrivance, and that the modification it 

 sustains in the case of Fishes and Reptiles is 

 for the purpose of carrying into effect some 

 additional design. 



Considering the cranium as a capsule for the 

 brain, its form is necessarily determined by the 

 extent to which that organ is developed in the 

 several classes of animals ; while, at the same 

 time, the nature of its organization is in harmo- 

 nious correspondence with their habits, and with 

 the external circumstances by which they are 

 surrounded. J3y pursuing this inquiry from the 

 lowest to the highest animals, it will be per- 

 ceived that, as respects both form and struc- 

 ture, additions are made in proportion as the 

 endowments are of a more and more exalted 

 character; and further, that these successive 

 changes of structure are the changes which the 

 human skull itself experiences in its progress 

 from a foetal to an adult condition. 



The rudimentary part of the most elaborate 

 cranium is a sac consisting of two membranes 

 and an intervening gelatinous iluid ; in the 

 next step of the formative process, this gelati- 

 nous fluid gives place to cartilage. A deposi- 

 tion of earthy matter in this cartilaginous nidus 

 gives it firmness, but breaks up the sac into 

 isolated unimited patches. These isolated 

 patches coalesce in definite numbers, and thus 

 establish a secondary and less numerous divi- 

 sion of ununited parts; these, in their turn, 

 approach and combine with each other, form- 

 ing a solid case of bone; and lastly, this solid 

 case resolves itself into two tables of different 

 structure, and a still further differing connect- 

 ing medium. In each and all of these states 

 through which the crania of the Mammalia 

 pass there is presented to us a type of the skull 

 in some lower animal. 



In Fishes the cranium is little more than a 

 tubular continuation of the spine through the 

 head to contain a similar prolongation of the 

 medulla spinalis. These, however, are not in 

 contact. A mass of reticulated membrane, 

 holding in its cells a gelatinous iluid, forms the 

 real superior investment of the brain ; while 

 the superjacent parietes are designed to afford 

 an extensive origin to the muscles of the body; 

 and as these muscles increase, so does the sur- 

 face of their attachment. For this purpose it is 

 that the ossific deposits remain ununited, that, by 

 being simply in juxtaposition, or at most over- 

 lapping each other, they may unfold them- 

 selves, and thereby admit of the head being 

 at all times in proportion to the rest of the 

 body. 



In Reptiles the skull is still further deve- 

 loped. It is charged more with earthy than 

 with animal matter; and this being loosely 

 distributed, tough spongy bones are the result. 

 The tardiness of their circulation does not 

 favour the combination of the individual por- 

 tions, and the bones are therefore for the most 

 part loose, although some of them unite by a 

 species of anchylosis in the direction in which 

 defence is required. ^ 



In Birds the character both of form and 

 structure is greatly changed ; light, fragile, and 

 compact, it is (by reason of the high Mate of 

 vitality which prevails) so rapidly and com- 

 pletely ossified over its entire surface as to 

 afford no evidence, or but a very slight one, of 

 its original subdivision. In conformity with 

 the development of the brain, it extends itself 

 backwards, to each side, and upwards as well 

 as forwards, thus constituting a considerable 

 portion of the entire head. 



In Mammiferous animals the skull is more 

 compact than that of Reptiles and more diffuse 

 than that of Birds. Its elementary portions 

 unite so as to form a determinate number of 

 bones which are either dovetailed together by 

 the interlacement of crooked processes with 

 which their edges are liberally studded, or flow 

 into each other so as to exhibit no trace of their 

 junction. Its structure is made up of two 

 osseous lamella;, called an inner and an outer 

 table, which are united by an areolar ossific 

 tissue, termed diplo'c, that adds greatly to the 

 defensive properties of the skull. 



The Cranium (in human anatomy) is a hollow 

 bone of an ovoid figure ; elongated from be- 

 hind forwards ; narrower before than behind ; 

 compressed on the anterior part of its sides ; 

 surmounting the face and spine, and projecting 

 considerably beyond the latter. It contains in its 

 parietes the organs of hearing, and contributes 

 to form the orbits, the nostrils, and the face. 



The dome-like upper portion is termed the 

 calvaria, and the lower part is the base. The 

 former presents the synciput in front, the occiput 

 behind, the vertex or brcgma, (j3p%/Aa, from 

 @F*x, u > ir >) above, and the temples on the 

 sides. 



Placed at the summit of the body and des- 

 tined to contain the brain, the skull is pierced 

 at its base by numerous foramina for the trans- 

 mission, 1st, of the nerves which establish the 

 communication between the brain and other 

 organs ; and 2dly, of the vessels which supply 

 the brain and its membranes. 



From the inferior surface of the cranium, be- 

 tween its anterior and middle thirds, there de- 

 scend two columns which limit posteriorly the 

 boundaries of the face ; so that it is anteriorly 

 to these columns that it contributes to form the 

 orbits and the nose, and consequently there the 

 bones which enter into the composition of the 

 face are fixed to it. Hence the surface of that 

 part is very irregular, presenting, in addition to 

 the foramina, depressions and elevations, sulci 

 and processes indicative of the articulation of 

 bones and the lodgement of other organs. 

 Posteriorly, between its middle and posterior 

 thirds, the base of the cranium overtops the 

 spine, and a great opening there establishes the 

 continuity of the vertebral canal with the inte- 

 rior of the skull ; and the muscles which move 

 the head and maintain its equipoise being at- 

 tached around, but especially behind this open- 

 ing, the skull iy strongly marked in that direc- 

 tion. The intermediate space or middle third 

 is above the pharynx, offering, centrally,* plain 

 surface to form the roof of that cavity, and, 



