86 VERTEBRATE ZOOLOGY 



The two median pieces are the first and second basibranchials,— 

 the first being the median cartilaginous bar which joins the hyoid 

 with the first branchial arch, and the second a slender median 

 bone just posterior to the first branchial arch. 



Exercise 22. Draw the mandible and the hyobranchial apparatus 

 on a scale of 2 . 



Thoroughly clean the remainder of the skull and the anterior 

 portion of the spinal column. Disarticulate the head from the 

 spinal column. 



Observe the flatness of the skull and its compactness. The 

 cranial bones and cartilages are intimately joined with the remain- 

 ing bones of the visceral skeleton, the whole forming so closely 

 knit a structure that its various parts are not easy to separate 

 from one another. At the hinder end is the foramen magnum, the 

 large opening through which the spinal cord enters the brain 

 cavity; at the forward end are two rows of small teeth. The 

 cranial portion of the skull, which incloses and protects the brain 

 and the special sense organs, forms the medial and larger part of 

 it. The upper jaw and other portions of the visceral skeleton 

 occupy the two lateral areas. 



This part of the skull, like the lower jaw and the hyobranchial 

 apparatus which have already been studied, is composed very 

 largely of cartilage. Of the bones present the greater number are 

 membrane bones, these being mostly thin plates which cover the 

 cartilages and which arch over the spaces between them. A few 

 cartilage bones are present, however, in the hinder part of the skull. 



The bones and cartilages of the cranium may be divided into two 

 groups : ( 1 ) those forming the cranium proper, or brain case ; and 

 (2) those forming the capsules of the special sense organs. 



The cranium proper contains the following bones: Surround- 

 ing the foramen magnum, except a small space on the dorsal and 

 ventral sides, are the paired exoccipital bones. They are cartilage 

 bones ; the remainder of the occipital region does not ossify but 

 remains cartilaginous. Each exoccipital bears on its posterior sur- 

 face an articular process, the occipital condyle, by which the skull 

 articulates with the spinal column. Forming the roof of the era- 



