III.] 



THE CRANIAL SKELETON. 



capsule on each side of it, we are reminded of the permanent 

 condition of the skull in the Lamprey. 



In that the trabeculae cranii first diverge and then con- 

 verge, meet, and so enclose what becomes the pituitary space, 

 we find a temporary condition of which large traces remain 

 even in adult Ophidians, where the cartilaginous trabeculae 

 persist as two rods, one on each side of the para-sphenoid. 



FIG. i2i. THE SKULL, ANTERIOR PART OF SPINAL COLUMN, AND BRAN- 

 CHIAL BASKET OF THE LAMPREY. 



(From Miiller and Owen.} 



-lit, auditory capsule ; b, cartilaginous basket, connected above with the side cl 

 the vertebral column, with seven complete and descending arches united by 

 transverse bands (between which the gill-openings are seen) and sheltering the 

 heart and pericardium at the part where the letter b is placed ; e, ethmo- 

 vomerine cartilage ; h, rudiment of the hyoid ; , neural arches ; P, palato- 

 quadrate (or pterygo-palatine) arch, the hinder pier of which represents the 

 suspensorium, though there is no lower jaw. 



Before the development of the third visceral arch we have 

 the permanent condition of the Lamprey, which is always 

 destitute of a mandible ; and when, in man, the visceral arches 

 successively arise, we have transitorily represented the piscine 

 condition, where the solid axes of seven or more such arches 

 form the mandible, the hyoidean arch, and the successive 

 branchial or gill-bearing arches. 



In the development of the palatal, or second visceral arch, 

 there is sketched .out a condition permanent in Sharks, where 

 the lower jaws bite against a cartilaginous pterygo-palatine 

 arcade which takes the place of an as yet undeveloped bony 

 upper jaw. This condition is essentially similar to the struc- 

 ture of the Sturgeon, where a comparatively minute pair ot 

 jaws (mandibular and pterygo-palatine respectively) are 

 suspended at the end of a disproportionately large suspen- 

 sorial structure. 



In that stage of the human skull which precedes ossification 

 we have a reminder of the exclusively cartilaginous structure 

 of certain Fishes. 



In the distinct ossific origin of the elements of the petrous 



