MOEPHOLOGY OF THE SKULL. 73 



which it joins the body of the jaw being about 160°. During the first year 

 osseous union takes place at the sj'mphysis between the segments of the jaw. 

 In this and subsequent yeai's the body becomes deeper, thicker, and longer, the 

 ramus and the neck of the condyle lengthen, and the angle at which the ramus 

 joins the body becomes less obtuse, till in the adult it is nearly a right angle. 

 In old age, consequent upon the loss of the teeth and the absoi-ption of the alveolar 

 margin, the body becomes shallower, and the angle is again increased. 



The hyoid bone has five points of ossification — one for the body, and one for 

 each of its great and small cornua. The ossification begins in the great cornua 

 and body in the last month of foetal life. Ossification of the small cornua 

 begins in the first year, but osseous union with the body only occurs after middle 

 life. The stylo-hyoid ligaments are occasionally ossified in some part of theii- 

 extent. 



GENERAL MORPHOLOGY OP THE BONES OP THE HEAD. 



The cii'cumstances which contribute most to modify the fonn of the human 

 skull and the condition of its component bones, as compared with that of 

 animals, are — 1 st, the proportionally large size of the brain and the coiTesponding 

 expansion of the cranial bones which enclose it ; 2nd, the smaller development 

 of the face as a whole, and especially of the jaws, which brings the facial bones 

 almost entii-ely under the fore part of the brain-case, instead of in front of it, as 

 occurs in all animals, with the partial exception of the anthropoid apes ; and 3rd, 

 the adaptation of the human skeleton to the erect posture, which, as regards 

 the head, is attended -ttdth the sudden bend of the basicranial axis at a consider- 

 able angle upon the line of the erect vertebral column ; and along with this tlie 

 advance of the occipito-vertebral ailiculation to such an extent as to make the 

 head neai-ly balanced on the upper extremity of the spine. The downward openings 

 of the nostrils, the forward aspect of the orbits and eyes, the neai'ly vei-tical fore- 

 head, and more or less oval-shaped face, are accompaniments of these human 

 peculiarities in the form of the head, which, together with those alreadj^ men- 

 tioned, stronglj' contrast with the smaller cranium and its strong crests of bone, 

 the larger projecting face and jaws, and the other characteristic featiu'es of the 

 skull in most animals. 



As regai-ds the condition of the individual bones, it is fuither to be remarked 

 that there is generally in the human skull a more complete consolidation or bony 

 union of the osseous elements than in animals, so that the whole number of 

 bones forming the cranium and face is least in man. Tlius. to mention only 

 some of the most marked examples of this difference among mammals ; the 

 frontal bone and the lower jaw frequently divided into two lateral poi-tions ; the 

 premaxillaiy very generally a separate bone from the maxillary ; the presphenoid 

 in many separate from the postsphenoid : the interpanetal from the occipital ; 

 the reduction in size of the squamosal, and its occasional separation from the 

 petrous ; the increase in the proportions of the tympanic portion or bulla : and 

 the veiy frequent development of honj elements connecting the hj-oid bone 

 dii-ectly with the skull. It is also worthy of observation that some of the conditions 

 now referred to as pcnnanent in animals exist as transitory stages of develop- 

 ment in the f cetus of man. 



Homolog-ies. — It would be out of place in this work to enter at any length 

 into the consideration of the homologies of the bones of the human skull, — a 

 subject which would require a minute reference to embryology and a wide range 

 of comparative anatomy. But as it may be useful at least to show the bearing of 

 such homological views on human anatomy, there has been introduced here a dia- 

 gi-ammatic figm-e of the bones of the fostal head (see fig. 64) and a table of tlieir- 

 nomenclature, in which an attempt is made to indicate the morphological relations 

 of the several bones to each other and to those of quadrupeds, and an explanation 

 is given of the signification of some of the terms applied by comparative ana- 

 tomists to these bones in connection with the names usuallj- given to them in 

 works on human anatomy. For fuller information on this subject the reader is 

 referred to the works of Owen, Goodsir, Huxley, Flower, St. George Mivart, and 

 others ah-eady quoted. 



