THE SKULL. 471 



conclusive argument against their being branchial arches, in that true 

 branchial arches hardly ever or perhaps never arise in this way. 



The sensg capsules. The most important of these is the auditory 

 capsule, which, as we have seen, fuses intimately with the lateral walls 

 of the skull. In front there is usually a cleft separating it from the 

 alisphenoid region of the skull, through which the third division of 

 the fifth nerve passes out. This cleft becomes narrowed to a small 

 foramen (fig. 327, V}. The sclerotic cartilage is always free, but pro- 

 foundly modifies the region of the cranium near which it is placed. 

 The nasal investment forms in Elasmobranchs (fig. 327, Na) a capsule 

 open below, and continuous with the ethmoid region of the trabeculae. 

 In most types however it becomes more closely united with the 

 ethmoid region and the accessory parts belonging to it. 



The cartilaginous cranium, the development of which has been 

 thus briefly traced, persists in the adult without even the addition of 

 membrane bones in the Cyclostomata, Elasmobranchii (fig. 327) and 

 Holocephali. Jn the Selachioid Ganoids it is also found in the adult, 

 but is covered over by membrane bones. In all other types it is 

 invariably present in the embryo, but becomes in the adult more or 

 less replaced by osseous tissue. 



Branchial skeleton. 



The most primitive type of branchial skeleton in any existing 

 form would appear to be that of the Petromyzonidse, which is deve- 

 loped in a superficial subdermal tissue, and consists of a series of bars 

 united by transverse pieces, so as to form a basket-work. It is known 

 as an extra-branchial system, and an early stage of its development in 

 the Lamprey is shewn in fig. 47. In the higher forms this system is 

 replaced by a series of bars, known as the branchial bars, so situated 

 as to afford support to the successive branchial pouches. Outside 

 these bars there may be present in some primitive forms (Elasmobran- 

 chii) cartilaginous elements, which are supposed to be remnants of the 

 extra-branchial system (rig. 327, Eac.Kr} ; while a series of membrane 

 bones is also usually added to them, which will be dealt with in a 

 separate section. The branchial bars are developed as simple carti- 

 laginous rods in the deeper parts of the mesoblast which constitutes 

 the primitive branchial arches. 



The position of the branchial bars in relation to the somatopleure and 

 splanchnopleure can be determined from their relation to the .so-called head 

 cavities. These cavities atrophy before the formation of the cartilaginous 

 braiicliial bars, but it will be observed (fig. 328), that the artery of each 

 arch () is placed on the inner side of the head cavity (pp). The cartila- 

 ginous bar arises at a later period on the inner side of the artery, and 

 therefore on the inner side of the section of the body cavity primitively 

 present in the arches. 



An anterior arch, known as the mandibular arch, placed in front 

 of the hyo-mandibular cleft, and a second arch, known as the hyoid 

 a\ck placed in front of the hyo-branchial cleft, are developed in aJJ 



