SKULL 



491 



bony case, which is admirably adapted for the 

 defence of the brain, for sheltering the organs of 

 special sense, and for being attached to the liga- 

 ments and muscles by which the skull is supported 



and moved on the 

 spine. At the 

 period of birth 

 most of the prin- 

 cipal bones nave 

 grown into appo- 

 sition with their 

 neighbours, form- 

 ing the sutures ; 

 but one large 

 vacuity remains 

 at the meeting- 

 point of the pari- 

 etal and frontal 

 bones, which is 

 termed the an- 

 terior fontanelle 

 (so called from the 

 pulsations of the 

 brain, which may 

 be here seen re- 

 2, posterior sembling the ris 



Fig. 2. Human Skull at Birth, 

 front above : 



1, anterior fontauelle ; _, ,. t 



fontanelle ; 3, sagittal suture ; 4, 4, ing of water at a 

 coronal suture; 6, lauibdoid suture ; spring or fountain. 

 6, 6, iMiietal bones ; 7, 7, two halve* r ,\. 

 of thefrontal bone, still ununited; lllere are two fon- 

 8, occipital bone. tanelles in the 



mesial line ( as 



shown in fig. 2), and two lateral fontanelles on 

 either side (as shown in fig. 3), which do not 

 close till the second year after birth, and some- 

 times remains open much longer. The deficiency 

 of the osseous brain-case at this position not only 

 facilitates delivery, but also acts to some extent 

 like a safety-valve during the first months of 

 infantile life, at which time the brain bears an 

 unusually large proportion to the rest of the body, 



Fig. 3. Human Skull at Birth (side view) : 

 1, 2, lateral fontanelles. 



and is liable to sudden variations of size from 

 temporary congestion, and other causes. The 

 sutures remain distinct long after the closure of 

 the fontanelles, and serve a purpose both in per- 

 mitting an increase of the size of the cranium by 

 the growth of the bones at their edges, and in 

 diminishing and dispersing vibrations from blows, 

 and thus contributing to the security of the brain. 



The number of centres of ossification in the skull 

 is tolerably constant ; each bone having a certain 

 nuiiiliiT. After the sutures have been formed, and 

 the skull has acquired a certain thickness, a pro- 

 cess of resorption commences in the interior of the 

 bones, and reduces the originally dense structure 

 to a more or lens cellular or cancellated state. 

 The interior thus altered is called the Diploe, and 

 by this change the weight of the skull is much 

 diminished while its strength is scarcely affected. 



The diploe usually begins to be apparent about 

 the tenth year, and is most developed in those 

 skulls which are thickest. A continuation of 

 the same process of resorption which causes 

 the diploe gives rise to the formation of the 

 cavities known as the frontal and sphenoid 

 sinuses. The formation of the diploe divides the 

 walls of the cranium into three layers viz. an 

 outer tough layer; an inner dense, brittle, and 

 somewhat glass-like layer, known as the vitreous 

 table or layer ; and the intervening cancellous 

 diploe. Diploe is absent from the cribriform plate 

 of the ethmoid bone and from the roof of the 

 orbital cavities, and thus these are the thinnest 

 parts of the cranium. The growth of the skull 

 after the seventh year proceeds slowly, but a slight 

 increase goes on to about the age of twenty. The 

 skull-bones are freely supplied with blood from 

 arteries which pass from the dura mater internally 

 and the pericranium externally, through the nu- 

 merous foramina observed on both surfaces ; the 

 blood being returned by veins which take various 

 directions. 



The fact that concussion of the brain scarcely 

 ever proves fatal, unless there is also fracture of 

 the skull, affords the most distinct evidence that 

 the skull is constructed in such a manner that so 

 long as it maintains its integrity it is able to 

 protect its contents from serious lesion. This mar- 

 vellous protective power is due to its rounded 

 shape, whereby its strength is increased, and in 

 consequence of which blows tend to glide off it 

 without doing material damage. Moreover, the 

 curved lines or ridges which may be traced round 

 the skull tend to strengthen it. The weakest part 

 of the skull is at the base. Herice, notwithstanding 

 its removal from exposure to direct injury and the 

 protection afforded by the soft parts, fracture takes 

 place more fre- 

 quently at the 

 base than at 

 any other part 

 of the skull, 

 fracture often 

 taking place 

 here even when 

 the skull was 

 not broken at 

 the partstruck. 

 There are two 

 points in the 

 architecture of 

 the bones of 

 the face which 

 deserve espe- 

 cial notice 

 viz. ( 1 ) the 

 great strength 

 of the nasal 

 arch ; and (2) 

 the immobility 

 of the upper 

 jaw, which is 

 fixed by three 

 buttresses the Fig. 4. Base of the Human Skull : 

 nasal, the zygo- 1, 1> hard palate; 2, 2, palate bones; 3, 

 jnatip nnrl tl> vomer ; 4, zygoniatic fossa; 6. basilar 

 process of the occipital bone ; 6, foramen 

 P S!L i magnum; 7, foramen ovale; 8, glenoid 



The base of fossa ; 9, external auditory foramen ; 10, 

 the skull carotid foramen of the left side ; 11, 

 whpthpr Rpon tyloid process : 12, mastoid process ; 18, 

 seen one of the Conrtj ,i e8 of the oc c ip| ta i bone. 

 from within or 

 from below, 



presents many objects of physiological interest 

 in relation to the nervous system. As seen from 

 within, the base presents on each side three fossa;, 

 corresponding to the anterior and middle lobes 

 of the cerebrum and to the cerebellum. These 



