218 AN INTRODUCTION TO ZOOLOGY 



small hole in the middle. Inter vert ebrally the chorda is not con 

 stricted, but retains its original size, so that in median longitudinal 

 section it appears as a series of diamonds strung together. The 

 centrum is finally completed by the growth around it of a thin 

 extension of the bases of the arches. Each vertebra is thus com- 

 posed of a number of parts which develop separately, but all come 

 from mesenchyme. Between the vertebrae the tunica skeletogena 

 is transformed into the strong fibrous intervertebral ligament. 



Skull. 



The skull of the dogfish remains in a cartilaginous con- 

 dition throughout life, and furnishes a very good example of a 

 primitive vertebrate skull which is not modified as in the higher 

 Craniates by the addition of bony structures developed either in 

 the cartilages or the surrounding membranes. It consists of a 

 cranium or brain case, with which are fused the olfactory and 

 auditory capsules, and to which are connected a series of seven 

 paired segmental visceral arches which originate as supporting 

 elements in relation to the perforations of the pharyngeal walls 

 known as the gill clefts. 



The chondrocranium of Scyllium, so called to indicate that it 

 remains cartilaginous throughout life, is shaped like a slightly 

 flattened oblong box within which lies the brain. Its floor and front 

 end are complete, and so also are the two long sides, save for a number 

 of small perforations through which nerves or blood-vessels pass. 

 The hinder end is wide open, leaving a large hole, the foramen 

 magnum, through which the brain is continuous with the spinal cord. 

 The cartilaginous roof is incomplete at the anterior end, where there 

 is situated a large elongated oval opening, the anterior cranial 

 fontanelle, closed, however, by a membrane. 



The hinder region of the cranium is formed by a ring of cartilage 

 surrounding the foramen magnum, and it is termed the occipital 

 region. It bears ventro-laterally of the foramen two rounded smooth 

 prominences, the occipital condyles, whereby the cranium articulates 

 with the front end of the vertebral column. The cranial floor 

 between the condyles is formed by the basal plate of cartilage which 

 runs forward as far as a small median perforation, the internal 

 carotid foramen, through which the similarly named artery enters 

 the brain case. The portion of the cranium immediately in front 

 of the occipital is known as the otic region, and it is greatly expanded 

 owing to the fact that it has fused with it the large auditory or otic 

 capsules in which are lodged the structures of the internal ear. 

 Clearly showing on the roof of the auditory capsule are two ridges 

 marking the position of the anterior and posterior semicircular 



