4 ALLEN'S NATURALIST'S LIBRARY. 



mouth are the two maxillse, along the margins of which are placed 

 the grinding- or cheek-teeth ; the two pre-maxillae, in which are 

 set the cutting- and the eye-teeth ; and lastly, the palatine bones 

 which form the roof of the mouth. Hinged on to the sides of 

 the cranium is the toothed mandible, or lower jaw, composed 

 of two halves, which may be solidly or loosely joined together 

 in the mid-line, or symphysis. Along the under surface of 

 the skull, there are, besides the great (often posterior) orifice 

 for the entrance of the spinal cord, numerous foramina, 

 or openings, for the passage of blood-vessels for the nourish- 

 ment of the brain, and of nerves which bring all parts of the 

 body into relation with the supreme directing centre. Con- 

 spicuous near its posterior part, on each side, is an ivory-like 

 capsule, the periotic bone, containing the essential organ of hear- 

 ing. Lying beneath the lower jaw is the hyoid arch, a slender 

 framework of bones, supporting the tongue and the upper end of 

 the windpipe with the organ of voice. In a few of the Monkeys 

 and Apes certain of the bones of this arch are much enlarged 

 and hollowed for increasing the volume of sound emitted by 

 them. On either side of the great opening which is so conspicu- 

 ous at the hinder part of the skull, for the reception of the spinal 

 cord, is a smooth kidney-shaped surface, called a " condyle." 

 These two condyles serve for the articulation of the first 

 segment of the back-bone to the cranium, and by the posses- 

 sion of this pair of condyles the Mammalian skull can always be 

 distinguished from that of Birds and Reptiles. The pieces of 

 which the back-bone are composed are named the -vertebra. 

 Those of the neck, the " cervical " vertebrae, are recognised by 

 having no true ribs attached to them, and are, in all Primates, 

 seven in number. Those of the back, or " dorsal " vertebrae, 

 may be distinguished by having articulated to them,on each side, 



