SL power equal to that by which they raise the lower jaw, and press it firm- 

 ly against the upper. 



The effort exerted from the arch to the base of the skull depends there- 

 fore, on two very different causes: the portion resulting from the action 

 of the elevators of the lower jaw, is equal to the effort exerted from be- 

 low upward, by this bone. After what has been stated, it would be use- 

 less to say any thing further of the manner in which the effort is trans- 

 mitted : we may merely observe, that the least powerful of these muscles, 

 the internal pterygoid, tends to draw the sphenoid downward, and pre- 

 vents this bone, fixed like a wedge with its base turned upward, from 

 being disengaged by the effort applied to it, by the bones between which 

 it is situated. 



The posterior, anterior, inferior and lateral faces of the sphenoid bone, 

 support, therefore, the whole effort of the bones of the skull and face, on 

 one another, when the top of the head being loaded with a heavy burden, 

 one presses, at the same time, something very firmly between the dental 

 arches. 



The anterior part of the body of the bone, containing the sphenoidal 

 sinus, is thin and very frail ; the posterior part, corresponding to the celia 

 turcica, is alone capable of resisting the effort which, I believe, it is des- 

 tined to sustain*: hence, it is at this point, that ossification begins, and 

 this confirms the observation of Kerkringius that the spot at which bones 

 begin to ossify, is that on which they have to bear the greatest effbrt; 

 hence the alae majores, by means of which the greatest part of the efforts 

 which the body of the sphenoid has to support, arise from the lateral part 

 of its posterior half, by an origin of considerable size, and which is fur- 

 ther increased by the base of the pterygoid processes which arise from 

 its lower part. 



I have, in this inquiry, purposely avoided mentioning the support which 

 the head receives from the vertebral column, and which, in the case un- 

 der consideration, is of use merely in preventing it from yielding to the 

 law of gravitation. If the bones of the skull and of the face had passed, 

 during the effort which they sustain, on the circumference of the fora- 

 men magnum; this aperture would have been incapable of increasing its 

 dimensions, and this would have been attended with the most serious in- 

 conveniences. 



The name given by the ancients to the braje whose principal use has 

 just been explained, is composed of spheno^which means a wedge, and 

 eidos, which signifies resemblance, and would lead one to think, that they 

 were not ignorant of its uses. From its situation, at the middle and in- 

 ferior part of the skull, and from its various connexions with the bones 

 which form this osseous case, it is to them of the same use as the key- 

 stone of arches, with regard to the different parts of which they are 

 formed. The numerous connexions required for this purpose, account 



* The sphenoidal sinus is prolonged, it is true, into this posterior part of the body of 

 the bone, in persons considerably advanced in years; but the parietes of this portion of 

 its cavity are of considerable thickness. The anterior part of the basilary process of 

 the occipital bone, is ihen firmly united to the sphenoid, and maybe considered as form- 

 ing a part of that bone, from which it cannot be detached. The cranium of an old man, 

 in this respect, resembles that of several quadrupeds, in which the union of the sphe- 

 noid to the occipital bone takes place so early, thatthese two bones might well be con- 

 sidered as forming but one. 



