GREGORY: FISH SKULLS 43 S 



the roof of the ethmoid, frontal, parietal and supraoccipital bones. A circle formed by the 

 orbit is superposed upon the triangle; it touches the hypothenuse above and extends below 

 the base-line. The orbit divides the neurocranium into four regions with widely different 

 functions. The first and most essential part is the endocranial vault or braincase proper 

 behind the orbits, the second is the interorbital bridge, the third is the ethmo-vomer block and the 

 fourth is the keel-bone or parasphenoid. In a general way the endocranial vault has eight 

 great functions: (1) it must be so built as to resist the thrusts of the backbone in the rear and 

 of the water in front; (2) it must resist the wrenching force of the great epaxial muscles of 

 the back and of the anterior dorsal fin muscles; (3) it must afford a firm anchorage for the 

 hyomandibular and thus resist all the wrenching, pulling and pushing forces that come from 

 the jaws and their muscles and from the struggles of the living prey; (4) it must afford a 

 support and anchorage for the shoulder-girdle and thus withstand a part of the strains 

 occasioned by the various muscles that are attached to the cleithrum, coracoid-scapula, 

 supracleithrum and posttemporal; (5) it must afford support to the often powerful muscles 

 that operate the opercular flap and to others that operate the branchial apparatus; (6) 

 it must at the same time protect from jarring the sensitive brain and cranial nerves that are 

 lodged within it; (7) it must also protect from any disturbance the extremely delicate sen- 

 sory-equilibrating apparatus, including the semicircular canals, the vestibule, the otolith, 

 etc.; (8) it must afford protection to the eyeballs and provide a myodome for several of their 

 muscles. 



The second functional region, or interorbital bridge, sometimes aided by the interorbital 

 septum and orbitosphenoid bone, must obviously be strong enough to resist the pressure 

 of the water above and in front; it must brace the ethmo-vomerine block in front and trans- 

 mit some of its thrusts to the endocranial vault behind. It must afford a channel for the 

 olfactory nerves; and it must help to suspend the eyes and keep them in exact alignment. 



The ethmo-vomerine block receives on its upper slope the thrusts of the water and 

 transmits part of them to the interorbital bridge, part to the keel-bone below; the median 

 and lateral ethmoids together give secure lodgment to the delicate olfactory capsules; the 

 lateral ethmoids form the anterior margin of the orbits and receive the thrusts of the pala- 

 tines, maxillae, premaxillse and vomers. 



The keel-bone, or parasphenoid, ties the ethmo-vomerine block to the base of the brain- 

 case, forms the roof of the mouth and the floor of the interorbital septum, gives off dorso- 

 lateral wings or struts, which often form a secondary anterior wall of the braincase proper, 

 affords attachment laterally to the elongate adductor arcus palatini muscle and forms the 

 floor of the myodome in which certain of the eye-muscles are lodged. In embryonic and 

 larval stages of typical teleosts the eyes are very large and the head very short. Conse- 

 quently the ventral eye-muscles grow backward into the floor of the cranial vault, which 

 yields to them and forms a funnel-like tunnel. As growth proceeds and the distances 

 between this incipient myodome and the centers of the orbits increase, the eye-muscles 

 retain their posterior connections and the myodome takes on its adult form. 



To consider the cranial vault in somewhat greater detail, it seems evident that at least 

 in typical fishes the final anchorage or base of the entire system is constituted by the 

 rear portions of the basi- and ex-occipital bones, as well as the centrum, neural arches and 

 anterior zygapophyses of the first vertebra. In the typical acanthopts the greatest con- 

 centration of bony tissue in this region is seen around the three occipital condyles (Fig. 299), 



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