XIII . PHYLUM CHORDATA 149 



<a large rounded aperture — the foramen mci()num — through which 

 the spinal cord, contained in the neural canal and protected by the 

 neural arches of the vertebrae, becomes continuous with the brain, 

 lodged in the cranial cavity. Below this, on either side is an 

 articular surface — the occipital condyle — for articulation with the 

 spinal column, and between the two cond,yles is a concavity, like 

 that of the vertebral centra, containing notochordal tissue. 



A number of smaller apertures, or foramina, chiefly for the 

 passage of nerves, perforate the wall of the skull. Behind and to 

 the outer side of the anterior fontanelle is an aperture through 

 which the ophthalmic branches of the fifth, and seventh nerves 

 leave the skull. Piercing the inner wall of the orbit are foramina 

 through which the optic or second pair of cerebral nerves (opt.), the 

 oculomotor (oc), or third, the pathetic, or fourth (path.), the tri- 

 geminal, or fifth, the abducent, or sixth, and the facial, or seventh, 

 gain an exit from the interior of the cranial cavity. Just behind 

 the auditory region is the foramen for the glossopharyngeal, and in 

 the posterior wall of the skull, near the foramen magnum, is the 

 foramen for the vagus. 



In close connection with the cranium- are a number of cartilages 

 composing the visceral arches (Figs. 813 and 814). These are in- 

 complete hoops of cartilage, mostly segmented, which lie in the 

 sides and floor of the mouth-cavity or pharynx. The first of these 

 forms the upper and lower jaws. The upper jaw, or palatoquadrate 

 {pal. q.), consists of two stout rods of cartilage firmly bound to- 

 gether in the middle line and bearing the upper series of teeth. 

 The lower jaw, or Meckel's cartilage (mck.), likewise consists of two 

 stout tooth-bearing cartilaginous rods firmly united together in the 

 middle line, the union being termed the symphysis. At their outer 

 ends the upper and lower jaws articulate with one another by a 

 movable joint. In front the upper jaw is connected by a ligament 

 with the base of the skull. 



Immediately behind the lower jaw is the hyoid arch. This 

 consists of two cartilages on each side, and a mesial one below. 

 The uppermost cartilage is the hyomandihdar (hy.mn.): this 

 articulates by its proximal end with a distinct articular facet 

 on the auditory region of the skull ; distally it is connected 

 by ligamentous fibres with the outer ends of the palatoquadrate 

 and Meckel's cartilage. The lower lateral cartilage is the cerato- 

 hyal (cer. hy.). Both the hyomandibular and ceratohyal bear a 

 number of slender cartilaginous rods — the branchial rays of the 

 hyoid arch. The mesial element, or hasihyal (h. hy.), lies in 

 the floor of the pharynx. Behind the hyoid arch follow the 

 branchial arches, which are five in number. Each branchial arch, 

 with exceptions to be presently noted, consists of four cartilages. 

 The uppermost of these— pharT/ngobranchial (ph. hr.'^-ph. br.^)—\ie 

 in the dorsal wall of the pharynx, not far from the spinal column ; 



