I400 A MANUAL OF ANATOMY 



The blind olfactory pits now grow dorsalwards along the roof of the stomo- 

 daeum, and so give rise to the nasal sacs, which represent primitive nasal 

 fossae. As each pit grows dorsalwards it is deepened by the formation of a 

 groove, called the nasal groove, which extends to the roof of the stomodaeum. 

 This groove is bridged over by ectoderm, and is thus separated from the 

 corresponding olfactory pit. When the maxillary process (previously joined 

 by the lateral nasal process) unites with the globular process, the nasal groove 

 is obliterated, and the olfactory pit is cut off from the stomodaeum. By the 

 development of the hard palate on either side, the ectodermic covering of 

 the nasal groove disappears. The posterior part of this covering constitutes 

 the naso-stomodesal membrane. When this membrane ruptures on either 

 side, a dorsal aperture, called the primitive posterior naris, is formed, hkewise 

 on either side, which establishes a fresh communication between the olfactory 

 pit (blind end) and the stomodaeum on the cephalic side of the corresponding 

 palatal shelf. 



The olfactory epithelium is developed from the upper part of the nasal pit, 

 and it is separated from the olfactory bulb by the cribriform plate of the 

 ethmoid bone. The ectodermic cells of the upper part of the nasal pit con- 

 stitute a neuro-epithelium, and each cell is prolonged into a slender process, 

 which is an axis-cylinder process, or axon. These axons form the olfactory 

 nerve-filaments, which are non-medullated, and they pass through the foramina 

 in one-half of the cribriform plate. Having reached the under surface of the 

 olfactory bulb, they enter the bulb and break up into arborizations, which 

 intermingle with the arborizations of the mitral cells of the bulb. 



For the Development of the Organ of Jacobson see Index. 



Rhinencephalon. — ^The rhinencephalon or olfactory brain repre- 

 sents the olfactory region of each cerebral hemisphere, and is feebly 

 developed in man. It consists of the following closely associated 

 parts : 



Morphologically considered, the cerebral hemisphere is composed of three 

 parts — namely, the stem, rhinopallium, and neopallium. The stem or stalk is 

 formed by the corpus striatum; the rhinopallium consists of the parts which 

 compose the rhinencephalon; and the neopallium represents the remainder of 

 the hemisphere. 



Corpus Callosum. — The corpus callosum connects the two cerebral 

 hemispheres. It is situated at the bottom of the great longitudinal 

 fissure, and extends nearer to the front than back of the hemispheres. 



It is arched and thicker in front and behind than at the centre, 

 its greatest thickness being posteriorly, where more fibres cross in 

 it than elsewhere, on account of there being more of the hemisphere 

 behind it than in front of it. 



The superior surface is related to the falx cerebri, but is in contact 

 with it only posteriorly. It is covered by a thin layer of grey 

 matter, and presents a transversely-striated appearance, indicative 

 of the direction of its fibres. In the median line there is a slight 

 antero-posterior furrow or raphe, and on either side of this there is a 

 white longitudinal band, called the stria longitudinalis mesialis. 

 The two striae longitudinales mesiales, right and left, are sometimes 



