THE PERCH 219 



Exercise 24, Make a drawing of the ventral aspect of the gill and 

 hyoid arches on a scale of 2 ; carefully label their various parts. 



Remove the bones of the visceral skeleton and study the 

 cranium. 



The cranium is 11 complicated structure, made up of a large 

 number of bones and cartilages. The bones are so tightly bound 

 together that the sutures are often indistinguishable until the 

 skull has been soaked a long time or boiled in a solution of dilute 

 caustic potash. 



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 fiat 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. The basioccipital is ventral ; 

 its concave posterior surface articulates 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 supraoccipital is dorsal and bears a broad median spine. On 

 each side of these bones and forming the posterolateral part of 

 the cranium is the auditory capsule. 



Exercise 25, 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 supraoccipital are the two large frontal bones ; these form 



