OPOSSUM 



B VENTRAL VIEW 



ceratobranchial I 



PLATYPUS 



LATERAL VIEW 



-trachea 



thyroid cricoid 



hypohyal 



ceratohyal 



ceratobranchial I 



ceratobronchiols II, 111 (thyroid 



cartilage) 



cricoid cartilage (ceratobranchial IV) 



U ECHIDNA 



Figure 3-11. Hyoid apparatus of a marsupial and the monotremes. 

 A, opossum; B and C, Platypus; D, Echidna. 



adult it is largely replaced by endochondral bone, and in 

 the embryo it may be represented by several separate chon- 

 drification (cartilage forming) centers. First, the fully formed 

 chondrocranium, at the time of the beginning of ossification 

 of the chondral bones, will be described, then its origin and 

 development. 



The chondrocranial skeleton of man, cat, and the rabbit, 

 as representative placentals, will be described. The termin- 

 ology is extremely complex but can be simplified. 



P/ocenfo/ mammah The fully formed chondrocranium 

 of man (Figure 3-12) underlies the brain and encloses the 

 nasal passages and the inner ear. The otic capsules, enclosing 

 the inner ear, are perhaps the most apparent structures. These 

 lie to either side above and forward to the large foramen mag- 

 num. There is a large internal auditory meatus, a deep subar- 

 cuate fossa, and a small endolymphatic foramen, as already 

 described for the bony ear capsule. Below the posterior part 

 of the internal meatus, on the ventral inner aspect of the 

 capsule, is a small perilymphatic fenestra. The otic cap- 

 sule consists of a large cochlear portion anteroventrally, an 



intermediate vestibular part, and a dorsoposterior canali- 

 cular region. 



The otic capsules are connected by the basal plate in 

 front of (below) the foramen magnum and behind (or above) 

 by the occipital arch and tectum posterior. .'Kbove and be- 

 hind the capsules are the parietal plates. Behind the otic 

 capsules are a Hne of foramina. In the occipital arch, next 

 to the foramen magnum, is the hypoglossal foramen, through 

 which the two roots of that nerve pass. Lateral to the 

 hypoglossal foramen is the jugular foramen, marking the 

 ventroanterior margin of the metotic fissure, which in less 

 developed chondrocrania separates the occipital arch from 

 the otic capsule. Above and behind these foramina is the 

 occipitocapsular fissiu-e and above this a mastoid foramen, 

 a remnant of the parietocapsular fissure. 



Laterally the otic capsule projects around the dorsal 

 and posterior margins of the tympanic fossa as a parocci- 

 pital process. An anterior extension from this paroccipital 

 process of the capsule is called the tegmen tympani ("roof of 

 the ear cavity"). The outer opening of the facial canal lies 

 medial to the tegmen tympani, which in the course of fur- 

 ther development extends forward and medially to form the 

 lateral prefacial commissure dividing this outer opening in- 

 to an upper hiatus facialis and a lower foramen primitivum. 

 The tegmen forms the outer wall of the facial sulcus of the 

 ossified petrosal mass. The prefacial commissure is the area 

 of cartilage lying dorsal to the facial canal between the inner 

 portal of that canal and the hiatus facialis. 



Below the tegmen tympani lies the fenestra vestibuli 

 (fenestra ovale); into it fit the cartilaginous stapes, and below 

 this, at an early stage, is the fenestra perilymphaticus. Dur- 

 ing the course of development, the fenestra perilymphaticus 

 becomes subdivided by a process (processus recessus) into 

 an outer fenestra cochleae (fenestra rotunda) and an inner 

 foramen perilymphaticus. 



The hyoid arch is represented by a long strand, Reichert's 

 cartilage, attached to the posterior margin of the tympanic 

 fossa just anterolateral to the notch of the primitive stylo- 

 mastoid foramen of the facial nerve canal. 



The basal plate, lying between the otic capsules, con- 

 tinues forward, bearing on its dorsal aspect a cartilaginous 

 dorsum sella and in front of this a hollow for the pituitary- 

 gland, the sella turcica. To either side of the sella area is a 

 ventrolateral projection, the basitrabecular process, or the 

 processus alaris; to the tip of this is fused (or closely bound) 

 the laterally projecting ala temporalis, penetrated by the 

 fenestra rotundum. A posteriorly directed process from the 

 processus alaris, the alicochlear commissure, extends back 

 to fuse with the cochlear mass and thus encloses the internal 

 carotid foramen and separates it fi-om the rest of the ali- 

 sphenocapsular fissure, which also gives rise to the anterior 

 lacerate fissure of the bony skull. The pterygoid cartilage 

 is suspended from the lamina pterygoideus of the ala tem- 

 poralis by a mesenchyme strand. The pterygoid cartilage 

 gives rise to the hamular part of the pterygoid bone. 



DEVELOPMENT OF THE HEAD SKELETON • 51 



