THE PERCH 59 



skull has been boiled in a dilute solution of caustic potash ; the car- 

 tilages are the remains of the chondrocranium, the primitive carti- 

 laginous cranium which forms the skull of the young animal and in 

 the adult fish is partly replaced by bone. As in the case of the vis- 

 ceral portion of the skull, the bones of the cranium fall into two 

 categories : the cartilage bones, which develop in and replace the 

 primitive cartilage; and the membrane bones, which develop in- 

 dependently of the cartilage in connective-tissue membranes and 

 in the dermis of the skin. 



Two distinct regions are present in the cranium: (i) the cra- 

 nium proper, which is the brain case and comprises the bones and 

 cartilages in the medial portions of the skull ; and (2) the capsules 

 of the special sense organs, which protect the optic, auditory, and 

 nasal organs. 



Observe the general character of the cranium. Note the small 

 brain cavity and the foramen magnum, the opening at the hinder 

 end of the cranium by which the spinal cord enters it. Note the 

 orbit, which occupies a large space on the side of the cranium ; 

 the portion of the cranium in front of it forms the nasal capsule ; 

 that back of it on each side is the auditory capsule. Note the two 

 flat spines which project back of the auditory capsule on each side. 



Study the posterior aspect of the cranium. The hinder end is 

 formed of four occipital bones, all of which are cartilage bones. 

 The basi-occipital is ventral ; its concave posterior surface articu- 

 lates with the centrum of the first vertebra. The two exoccipitals 

 inclose the foramen magnum between themselves; each bears a 

 broad articular surface which meets a corresponding one on the 

 neural arch of the first vertebra. The foramen of the vagus nerve 

 is in the exoccipital on each side. -The supra-occipital is dorsal and 

 bears a broad median spine. On each side of these bones and form- 

 ing the posterolateral part of the cranium is the auditory capsule. 



Exercise 33. Draw the hinder end of the skull on a scale of 2, 

 showing outlines of these bones. 



Study the dorsal aspect of the cranium. Extending in front 

 of the supra-occipital are the two large frontal bones ; these are 

 membrane bones which form the greater part of the roof of the 



