xxix BRANCHIAL ARCHES 375 



face is a large aperture, the foramen magnum, through which 

 the brain joins the spinal cord, and on each side of the 

 foramen is an oval elevation or condyle for articulation with 

 the first vertebra. 



In the human and other higher vertebrate skulls the 

 upper jaw is firmly united to the cranium, and the lower 

 alone is free. But in the dogfish both jaws (up. /, /. j) are 

 connected with the cranium by ligament (/-, Ig) only, and 

 each consists of strong paired (right and left) moieties, 

 united with one another by fibrous tissue. The posterior 

 end of the upper jaw presents a rounded surface, on which 

 fits a corresponding concavity on the lower jaw, so that a 

 free articulation is produced, the lower jaw working up and 

 down in the vertical plane, not from side to side like the 

 jaws of the crayfish. 



The visceral arches consist of six pairs of cartilaginous 

 half-hoops, lying in the walls of the pharynx (Fig. 99, B, 

 Br. A\ and united with one another below so as to form a 

 basket-like apparatus supporting the gills. The first of these 

 arches is distinguished as the hyoid, and is situated imme- 

 diately behind the jaws. It consists of two parts, a strong, 

 rod-like hyomandibular (Fig. 100, hy. m), which articulates 

 above with the auditory capsule, and is connected below by 

 fibrous tissue with the jaws, thus helping to suspend them 

 to the cranium : and a hyoid cornu, which curves forwards 

 inside the lower jaw, and is connected with its fellow of the 

 opposite side by a median plate which supports the tongue. 



The remaining five arches (br. a. i br. #.5) are called the 

 branchial arches. Each is formed of several separate pieces-, 

 movably united by fibrous tissue so as to render possible the 

 distension of the throat during swallowing. Both they and 

 the hyoid give attachment to delicate cartilaginous branchial 

 rays (br. r, br. r : Fig. 99, Br^ R] which support the gills. 



