84 



SKULL OF A 43-M.M. HUMAN FETUS. 



paraseptal process and its ostium is, accordingly, 

 shifted backward from its earlier position. It 

 would appear that the present position of this 

 portion of the duct in man, far behind the para- 

 scptal process, is consequent upon the disap- 

 pearance of the cartilaginous connection between 

 the parascptal process and the nasal septum, which 

 completes the anterior transverse lamina in the 

 lower forms. 



BASAL FISSURE. 



The basal fissure (fig. 2), which is represented by 

 a fenestra in most of the lower mammals, extends 

 throughout the entire length of the ectethmoid, 

 being continued forward as the incisura narina. 

 Its lateral border has recently been outlined in the 

 discussion of the lower border of the ectethmoid 

 and its medial border follows the lower margin of 

 the septum, being indented by the paranasal car- 

 tilages. The lateral and medial borders meet in 

 front of the confluence of the ectethmoid with the 

 septum. There is no such confluence behind, 

 as we have seen, the caudal extremity of the 

 ectethmoid being separated from the medialh 



lying septum by the very narrow cupulo-septal 

 fissure. This fissure runs forward to the caudal- 

 most junction of the ectethmoid with the main 

 stem of the chondrocranium. The basal fissure 

 is interrupted by the inward projection of the 

 alveolar process of the maxilla. 



TYPE OF CARTILAGE. 



Summarizing the type of cartilage in the 

 ethmoidal region, we note that it is younger than 

 that of the other regions of the chondrocranium. 

 It has long been known that this region of the 

 skull develops later than the others and accord- 

 ingly it shows a rapid growth at this stage. In 

 the region of the connections with the paraseptal 

 cartilage, and with the upper portion of the ect- 

 ethmoid and at the crista galli, the septum 

 shows a young type of cartilage. In the ect- 

 ethmoid, the roof and edges are, for the most 

 part, of young cartilage, while the two upper con- 

 cha? and the posterior part of the inferior concha 

 are lined with precartilage. Precartilage, too, 

 forms the process of the middle meatus and tips 

 the paraseptal and posterior maxillary processes. 



CARTILAGINOUS BRANCHIAL- ARCH SKELETON. 



MECKEL S CARTILAGE. 



Meckel's cartilage (figs. 1, 3, 5, 6, 31, 32, 34) 

 is a long rod of mature cartilage continuous above 

 with the head of the malleus and terminating 

 below in a tip, turned forward and inward, which 

 approaches very close to, but does not meet, its 

 neighbor of the opposite side. The cartilage is of 

 practically uniform thickness throughout, except- 

 ing at the angle near the tip, where it is flattened 

 from above downward and forward, and widened. 

 In front of this the cartilage rapidly narrows to a 

 point. There is a constriction just below the head 

 of the malleus. The two rods converge to form a 

 wide angle (figs. 3, 4), in which lies the floor of 

 the mouth. The shaft, when seen from the side, 

 as in figure 6, has a practically straight course 

 with a very faintly marked anterior concavity. 

 When seen directly from the front there is, just 

 above the angle, an inward bending, which makes 

 a gentle concavity outward and downward here. 

 Above this point the shaft appears straight from 

 this point of view. Throughout almost its entire 

 extent the cartilage (as shown in figures 5, 31, ami 

 34), is covered by the mandible. At the lateral 

 surface in the region of the ventral bend (fig. 32), 

 there is a very close approximation of the carti- 

 lage to the mandible, and here the cartilage is 

 undergoing change preliminary to ossification. 



The perichondrium here has become ossified. 

 The modified cartilage cells extend far into the 

 interior. The corresponding area, on the mandi- 

 ble, is seen in figs. 33 and 35. The relation of 

 Meckel's cartilage to the slender goniale has been 

 referred to. 



STYLO-HYOID ARCH. 



The cartilage of Reichert, representing the future 

 styloid process, has been described with the otic 

 capsule. 



HYOID CARTILAGE. 



The hyoid cartilage (figs. 5, 6, 45 46, 47) is 

 roughly semicircular in form, resembling closely 

 the mature bone. It consists of a body and paired 

 lesser and greater cornua. 



The body, cartilaginous, with an edging of 

 young cartilage in regions adjoining the cornua, 

 is oval in form when seen from the front, with a 

 rounded cranioventral face. The caudal face 

 presents a distinct transverse notch, as seen in the 

 mid-sagittal section (fig. 46). Just above this the 

 upper edge of the thyroid cartilage is confluent 

 with the hyoid. At the site of connection with 

 the lesser cornu the cartilage is somewhat pointed, 

 but there is no direct cartilaginous connection 

 here, a fine strand of membrane joining the two. 



