COMPARATIVE ANATOMY. 



which are formed a vast number of cells, 

 communicating with the throat by me ms 

 of the eustachian tube, anil filled with air, 

 instead of the bloody or medullary sub- 

 stance which occupies the diploe of ani- 

 mals The use of this structure in in- 

 creasing- the surface for attachment of 

 those large muscles, which belong to the 

 lower jaw, proboscis and neck, and in 

 augmenting the mechanical power of 

 these muscles, by removing their attach- 

 ments to a greater distance from the cen- 

 tre of motion, has been very ingeniously 

 explained by Camper. ((Euvres, torn. 2.) 

 These advantages are attained by the 

 cellular structure which we have just 

 described, without augmenting the weight 

 of the head, and this precaution is parti- 

 cularly necessary in the present instance, 

 as the head is on other accounts more 

 heavy and massy in this than in any other 

 animal. The air cells of birds, in gene- 

 ral, and particularly those which pervade 

 the cranium in the ostrich, eagle, and 

 owl, present examples of a similar for- 

 mation, attended with the same uses ; viz. 

 those of increasing the bulk and strength 

 of the bone, and diminishing its weight. 



A comparison of the human cranium 

 with that of animals will lead us to some 

 interesting conclusions. Daubenton fix- 

 ed on the situation of the foramen mag- 

 num occipitale as a point of compari- 

 son. He draws two lines, which inter- 

 sect each other in the profile of the scull : 

 one passes from the posterior margin of 

 the great foramen, (which, in almost all 

 mammalia, is also the superior one,) 

 through the lower edge of the orbit; 

 the other takes the direction of the 

 opening itself, beginning at its posterior 

 edge, and touching the articular surface 

 of the condyles. He determines, accord- 

 ing to the angle formed by the junction 

 of these two lines, the similarity or diver- 

 sity of the form of crania. 



This angle is, however, but an imper- 

 fect criterion ; for its variations are in- 

 cluded between 80 and 90 in almost all 

 quadrupeds, which differ very essentially 

 in other points. And small variations 

 occur in the individuals of one and the 

 same genus. 



The variations in the situations of the 

 occipital foramen are important,, when 

 viewed in connection with the ordinary 

 position of the animal's body In man, 

 who is designed to hold his body erect, 

 this opening is nearly equi-distant from 

 the anterior and posterior extremities 

 of the skull. The head therefore is sup- 

 ported in a state of equilibrium on the 



vertebral column The angle, formed 

 by the two lines mentioned by Dauben- 

 ton, is only of three degrees. 



Quadrupeds, which go on all-fours, 

 have the occipital foramen and condvles 

 situated farther back, in proportion as 

 the face is 1 elongated. That opening, in- 

 stead of being nearly parallel to the hori- 

 zon, forms a considerable angle with it : 

 which, measured according to Dauben- 

 ton, is of 9 j degrees in the horse. The 

 weight of the head in these animals is 

 not therefore sustained by the spine, but 

 by a ligament of immense strength, which 

 is either entirely deficient, or so weak as 

 to have its existence disputed, in the hu- 

 man subject. This ligamentum muchae, 

 or cervical ligament, arises from the 

 spines of the dorsal and cervical verte- 

 brae, (which are remarkably long for that 

 purpose,) and is fixed to the middle and 

 posterior part of the occipital bone. It 

 is of great size and strength in all qua- 

 drupeds, but most particularly in the ele- 

 phant ; where the vast weight of the 

 head, so much increased by the enor- 

 mous size of the tusks, sufficiently ac- 

 counts for its increased magnitude. It is 

 bony in the mule, probably on account of 

 the use which the animal makes of its 

 head, in disengaging and throwing up 

 the earth. 



Animals of the genus Simia and Lemur 

 hold a middle rank between man, who is 

 constantly erect, and quadrupeds, whose 

 body is supported by four extremities. 

 Their structure is by no means calculat- 

 ed, like that of man, for the constant 

 maintenance of the erect posture ; but 

 they can support it with greater facility, 

 and for a longer time, than other animals. 

 Hence, in the orang-outang, the occipital 

 foramen is only twice as far from the 

 jaws as from the back of the head ; so 

 that Daubenton's angle is only of 37. 

 It is somewhat larger in the other species 

 of Simiae, and measure 47 in the lemur. 



The general form of the cranium is 

 most materially influenced by the direc- 

 tion, and the various degrees of promi- 

 nence, of the facial bones. 



To determine this with greater preci- 

 sion, Camper instituted the facial line ; 

 the application of which is most minutely 

 explained in his posthumous work, "On 

 the natural Differences of the Features, 

 &c." Like Uaubenton, he draws on the 

 profile of the cranium two straight lines, 

 which intersect each other ; but in dif- 

 ferent directions from those of the French 

 anatomist. An horizontal line passes 

 through the external auditory passage 



