64 BOKES OF THE HEAD. 



The SINUSES are hollows within certain cranial bones, which communicate 

 with the nasal cavities by narrow orifices, and are named ethmoidal, frontal, 

 sphenoidal, and maxillary. The maxillary sinus begins to be formed about 

 the fourth month of foetal life ; the frontal, ethmoidal and sphenoidal first 

 appear during childhood, but remain of small size up to the time of puberty, 

 when they undergo a great enlargement. In advanced life they all increase 

 in size by absorption of the cancellated tissue in their vicinity. The 

 ethmoidal sinuses are numerous smaller spaces in the lateral masses of the 

 ethmoid bone. The frontal sinuses, formed in the substance of the frontal 

 bone, communicate with the middle meatus narium through the infundibula 

 of the ethmoid bone. The sphenoidal sinuses, hollowed out in the sphenoid 

 bone, and limited below and in front by the sphenoidal spongy bones, op n 

 anteriorly opposite the posterior ethmoidal cells. 



The maxillary sinus is of an irregular pyramidal form ; its apex points to 

 the malar tuberosity ; its sides are formed by the orbital and lateral plates 

 of the superior maxillary bone ; its internal wall, which separates it from 

 the nasal cavity, is formed by the maxillary, palate, and inferior turbinated 

 bones, and the uncinate process of the ethmoid ; an irregular gap or deficiency 

 being left between the uncinate process and the inferior turbinated bone, by 

 which the sinus opens into the middle meatus. The alveolus of one of the 

 molar teeth generally forms a marked projection in the floor of the sinus, at 

 its outer part. 



DEVELOPMENT OF THE SKULL. 



EARLY DEVELOPMENT OP THE HEAD. When the head of the embryo has become 

 so far developed as to rise completely out from the plane of the germinal mem- 

 Fig. 55. Fig. 55. MAGNIFIED SIDE VIEW OF THE HEAD 

 AND UPPER PART OF THE BODY OP AN 

 EMBRYO-CHICK OP THE FOURTH DAY (adapted 

 from Remak and Huxley). 



1, chorda dorsalis ; 2, three of the upper 



2 /A> / N. vJ^^ primitive cervical vertebrae; C 1 , first cerebral 



fi^X . ^\^ vesicle, with the nasal fossa below; C 2 , second 



part of the first cerebral vesicle, or thalamus 

 opticus, with the eye below it ; C 3 , the middle 

 cerebral vesicle; C 4 , the cerebellum, between 

 which and the cervical vertebrae is the medulla 

 oblongata, these two constituting the first and 

 second parts of the third cerebral vesicle. At 

 the upper part of the chorda dorsalis, where it 

 afterwards reaches the post-sphenoid, is seen the 

 rectangular bend of the middle of the cranium, 

 which takes place at the sella turcica ; and in 

 front of this, towards the eye, the pointed infun- 



dibulum ; V, the rudiment of the trigeminus nerve ; VII, the facial ; VIII, the vagus ; 

 IX, the hypoglossal ; in front and below these numbers respectively, first the upper and 

 lower jaw, or first branchial arch, with the first cleft, which becomes the meatus audi- 

 torius externus ; and lower down the second, third, and fourth branchial arches and 

 clefts in succession ; in front of these the aortic bulb attaches the heart ; between VII 

 and VIII, the auditory vesicle. 



brane, two curvatures forwards are observed, the posterior of which is at the 

 junction of the head with the spinal column, while the anterior is opposite the second 

 cerebral vesicle, and is so placed that the fore part of the skull is bent at right angles 

 to the back part. Behind the anterior curvature, the ventral margins of the dorsal 

 plates are thrown on each side into four processes, the branchial or visceral arches, 

 behind each of which is a fissure or branchial cleft in which the epithelial layer of 



