106 



SPECIAL ANATOMY OF THE SKELETON 



Anterior Surface. 



adjoining part of the nasal wall. 4. A premaxillary, for the front part of the alveolus, which carries 

 the incisor teeth and corresponds to the premaxillary bone of the lower animals. (5) A nasal, 

 that gives rise to the nasal process and the portion above the canine tooth. (6) An infravomerine, 

 that lies beneath the vomer and between the palatal and premaxillary centres. The premaxillary 

 centre is in close association with the development of the perpendicular plate of the ethmoid and 

 the vomer. According to Albrecht it. develops from two centres of ossification, each having an 



incisive tooth. The one possessing the mesal segment he calls the 

 cndognathion. The lateral segment he calls the mesognaihion, 

 while to the maxilla he gives the name exognatJiion. These seg- 

 ments are separated by five sutures. The failure of union of any 

 of these segments will lead to the various forms of cleft palate. 

 These centres appear about the eighth week, and by the tenth 

 week have become fused together so that the bone consists of two 

 portions, one the maxilla proper, and the other the premaxillary 

 portion. The suture between these two portions on the palate 

 persists until middle life, but is not to be seen on the facial surface. 

 This is believed by Callender to be due to the fact that the front 

 wall of the sockets of the incisive teeth is not formed by the pre- 

 maxillary bone, but by an outgrowth from the facial part of the 

 maxilla. The antrum appears as a shallow groove on the inner 

 surface of the bone at an earlier period than any of the other nasal 

 sinuses, its development commencing about the fourth month of 

 fetal life, and reaches its full size after the second dentition. The 

 sockets for the teeth are formed by the growing downward of 

 two plates from the dental groove, which subsequently becomes 

 divided by partitions jutting across from the one to the other. If 

 the two palatal processes fail to unite partially or completely, a 

 partial or complete cleft palate results. 



Articulations. With nine bones, two of the cranium, the fron- 

 tal and ethmoid, and seven of the face viz., the nasal, malar, 

 lacrimal, turbinated, palate, vomer, and its fellow of the opposite 

 side. Sometimes it articulates with the orbital plate of the 

 sphenoid, and sometimes with its external pterygoid plate. 

 Attachment of Muscles. To twelve the Orbicularis palpebrarum, Obliquus oculi in- 

 ferior, Levator labii superioris alaeque nasi, Levator labii superioris proprius, Levator anguli oris, 

 Compressor naris, Depressor alae nasi, Dilatator naris posterior, Masseter, Buccinator, Internal 

 pterygoid, and Orbicularis oris. 



Applied Anatomy. It is from the extreme thinness of the walls of the antrum that we are 

 enabled to explain how a tumor growing from the antrum encroaches upon the adjacent parts, 

 pushing up the floor of the orbit and displacing the eyeball, projecting inward into the nose, 

 protruding forward on to the cheek, and making its way backward into the zygomatic fossa 

 and downward into the mouth. 



Inferior Surface. 



FIG. 73. Development of 

 the maxillae. At birth. 



CHANGES PRODUCED IN THE UPPER JAW BY AGE. 



At birth and during infancy the diameter of the bone is greater in an antero-posterior than 

 in a vertical direction. Its nasal process is long, its orbital surface large, and its tuberosity 

 well marked. In the adult the vertical diameter is the greater, owing to the development of 

 the alveolar process and the increase in size of the antrum. In old age the bone approaches 

 again in character to the infantile condition; its height is diminished, and after the loss of the 

 teeth the alveolar process is absorbed, and the lower part of the bone contracted and diminished 

 in thickness. 



The Lacrimal Bone (Os Lacrimale). 



The lacrimal (lacrima, a tear) is the smallest and most fragile bone of the face. 

 There are two lacrimal bones. They are situated at the front part of the inner 

 wall of the orbit (Fig. 67), and resemble somewhat in form, thinness, and size a 

 finger nail; hence, they are termed the ossa unguis. Each bone presents for exami- 

 nation two surfaces and four borders. 



Surfaces. The external or orbital surface (Fig. 74) is divided by a vertical 

 ridge, the lacrimal crest (crista lacrimalis posterior), into two parts. The portion 



