96 



LECTURE V. 



the same relation to tlie mesencephalic bony girdle, which the par- 

 occipitals do to the epencephalic one behind : and they project out- 

 wards and backwards further than the par-occipitals, forming the 

 second strong transverse process at each side of the cranium. This 

 process is developed from the outer margin of the mastoid ; the inner 

 side of the bone is expanded, and enters slightly into the forma- 

 tion of the walls of the cranial or rather the acoustic cavity, its inner, 

 usually cartilaginous, surface lodging part of one of the semicircular 

 canals.* It is wedged into the interspace of the ex- and par-occi- 

 pitals, the petrosal, the ali-sphenoid, the parietal, the frontal and post- 

 frontal bones. The projecting process lodges above, the chief mucous 

 canal of the head, and below affords attachment to the epitympanic, 

 or upper, piece of the bony pedicle from which the mandibular, hyoid, 

 and opercular bones are suspended : its extremity gives attachment 

 to the strong tendon of the dorso-lateral muscles of the trunk. The 

 mastoid is ossified in and from the primitive cartilaginous wall of the 

 ci'anium. 



The basal piece of the third cranial cincture, which defends the 

 prosencephalon, is formed by the pre-spheiioid (centrum of the fron- 

 tal vertebra. Jigs. 30. 33. 9) already described as connate with or 



produced from the basi-sphenoid. The 

 sides of the prosencephalon are de- 

 fended by the orbito-sphenoids (neu- 

 rapophyses of the frontal vertebra, 

 ib. lo) : these are osseous plates, 

 usually of a square shape, sometimes 

 semicircular or semi-elliptic, as in the 

 Cod ; larger in the Malacopteri {Jig. 

 35. 10) but very small, usually, in 

 Acanthopteri, and sometimes repre- 

 sented by a descending plate of the 

 frontal, as in the Garpike, or by un- 

 ossified cartilage, as in Mail-cheeked 

 fishes* They are occasionally separated 

 from the pre-sphenoid by the ali-sphe- 

 noid, to which they are articulated be- 

 low and behind, whilst above they are joined to the frontal and post- 

 frontal, completing the anterior part of the lateral walls of the cranium. 



Disarticulated neural arch of frontal 



vertebra, viewed from behind : Gadus 



Morrhua. 



* The great cavity, ' otocrane,' which the ex-occipital, par-occipital, ali-sphenoid, 

 mastoid, and sometimes the parietal and supra-occipital form for the lodgment of 

 the cartilaginous or osseous pi-oper acoustic capsule, 'petrosal,' of the great labyrinth 

 of fishes, may be com])ared to the accessary cavity or orbit, which lodges the car- 

 tilaginous or bony capsule, ' sclerotic,' of the organ of vision. 



