68 



BONES OF THE HEAD. 



and produces a triangular depression at the site of the posterior fontanelle, which is 

 easily recognised by the finger of the accoucheur. 



The temporal bone, considered in respect of its development, consists of the 

 squamous and petro-mastoid parts, the tympanic plate, and the styloid process. The 

 squamous part, inclusive of the zygomatic process, is ossified from a single nucleus. 

 The petrous and mastoid portions are also formed from only one centre : bone is first 

 deposited on the anterior surface, over the cochlea, and there afterwards appear one 

 or two nuclei on the posterior surface, in connection with the formation of the 

 semicircular canals ; the different nuclei soon unite, and ossification extends into 

 the mastoid process. The tympanic plate appears in the membranous wall of the 

 tympanum and external ear as a slender arch of bone forming three-fourths of a ring, 

 inclosing the membrana tympani. The styloid process is formed, as already stated, 

 in the second visceral arch. 



Fig. 61. Fig. 61. SEPARATE PARTS OF THE TEMPORAL BONE 



OF A CHILD AT BIRTH. 



a, squamous part with the zygoma; b, the tym- 

 panic bone forming an imperfect ring open superiorly ; 

 c, the petrous and mastoid part ; c being placed on 

 the mastoid part : part of the cavity shown is the 

 tympanum. 



The sphenoid bone presents in infancy traces 

 of a natural division into a posterior or post- 

 sphenoid part, to which the sella turcica and great 

 wings belong, and an anterior or pre-sphenoid part, 

 to which belong the body in front of the olivary 

 process and the small wings, a division which is 

 found in many animals complete and persistent 

 through life. The first osseous nuclei of the post- 

 sphenoid division occur in the great wings, one on 

 each side, between the foramen rotundum and 

 foramen ovale, and spread thence outwards into the wing and downwards into the 

 external pterygoid process. The internal pterygoid processes arise from distinct 

 nuclei, and, although in the human subject they soon unite with the external 

 pterygoid, in other animals they remain separate, and are named by comparative 

 anatomists the pterygoid bones. 



Fig. 62. 



Fig. 62. OSSIFICATION OF THE SPHENOID BONE. 



A, sphenoid bone from, a foetus of three months, seen from above; 1,1', the greater 

 wings ossified ; 2, 2, the lesser wings, in which the ossification has encircled the optic 

 foramen, and a small suture is distinguishable at its posterior and inner side; 3, two 

 round granules of bones iu the body below the sella turcica, the rest being cartilaginous. 



B, copied from Meckel (Archiv, vol. i. tab. vi. fig. 23), and stated to be from a foetus 



