332 CHARLES CLIFFORD MACKLIN 



Regio occipitalis 



The occipital region has, in general, the form of a ring, whose 

 irregular circumference, stoutly built and steeply sloping ven- 

 trally, plate-like and gently shelving dorsally, encloses the lower 

 part of the posterior cranial fossa, the floor of which is perfor- 

 ated by an elongated fissure, the primitive foramen magnum 

 (fig. 1). Its appearance suggests the occipital bone, of which 

 it is the cartilaginous precursor. 



Ventrally the ring is completed by a dorsally concave plate of 

 cartilage, the basilar portion of the occipital anlage, which has 

 been described as the lowermost part of the planum basale. As we 

 have seen, the basilar portion is directly continuous cranially and 

 laterally with the regio otica; below it splits to form the condyloid 

 or lateral portions of the regio occipitalis, which spring downwards, 

 outwards and sUghtly backwards and enclose, with their deviat- 

 ing, flattened limbs, the incisura intercondyloidea, marking the 

 ventralmost part of the primitive foramen magnum. Reach- 

 ing the most caudal points of the primitive skull at the paired, 

 downwardly projecting ventral foraminal prominences (figs. 2 

 and 3), upon the site of the future condyles of the occipital bone, 

 they suddenly bend upwards and outwards, twist on their long 

 axes so that the inner surfaces, which before looked dorso- 

 medially now look principally cranially, and at the same time 

 they broaden ventrally and laterally, their outermost borders 

 coming to underlie and support the partes canaliculares of the 

 otic capsules. The lateral wing-like plate, which is thus formed 

 on each side, is really the ventral and narrowest part of the squa- 

 mous portion, and is known as the lamina alaris (Voit) (fig. 5). 

 Ventrally it terminates in the prominent processus paracondy- 

 loideus, (figs. 2, 3 and 4) which may be seen from the front pro- 

 jecting laterally from the outer surface of the condyloid portion; 

 dorsally . it broadens into the squama, which becomes steeper, 

 and swings medially to pass into the tectum posterius (fig. 5). 

 The upper border, after skirting the dorsal surface of the ear 

 capsule, passes backwards and inwards, being continuous above 



