620 THYROID GLAND. THYMUS GLAND. 



ORGANS OF SENSE. 



Eye. The eyeballs are of large size and well developed at birth. 

 The pupil is closed by a vascular membrane called the membrana 

 pupillaris, which disappears at about the seventh month. Sometimes 

 it remains permanently, and produces blindness. It consists of two 

 thin membranous layers, between which the ciliary arteries are pro- 

 longed from the edge of the iris, and form arches and loops by return- 

 ing to it again, without anastomosing with those of the opposite side. 



The removal of the membrane takes place by the contraction of 

 these arches and loops towards the edge of the pupil. The capsule 

 of the lensis extremely vascular. 



Ear. The ear is remarkable for its early development ; the 

 labyrinth and ossicula auditus are ossified at an early period, and the 

 latter are completely formed before birth. The only parts remaining 

 incomplete are the mastoid cells, and the meatus auditorius. The 

 membrana tympani in the foetal head is very oblique, occupying 

 almost the basilar surface of the skull ; hence probably arises a 

 deficient acuteness in the perception of sound. It is also extremely 

 vascular. 



Nose. The sense of smell is very imperfect in the infant, as may 

 be inferred from the small capacity of the nasal fossae, and the non- 

 development of the ethmoid, sphenoid, frontal, and maxillary sinuses. 



THYROID GLAND. 



The Thyroid gland is of large size in the foetus, and is developed 

 by two lateral halves, which approach and become connected at the 

 middle line so as to constitute a single gland. It is doubtful whether 

 it performs any especial function in foetal life. 



THYMUS GLAND. 



The Thymus gland* consists "of a thoracic and a cervical portion 

 on each side. The former is situated in the anterior mediastinum, 

 and the latter is placed in the neck just above the first bone of the 

 sternum, and behind the sterno-hyoidei and sterno-thyroidei muscles." 

 It extends upwards from the fourth rib as high as the thyroid gland, 

 resting upon the pericardium, and separated from the arch of the aorta 

 and great vessels by the thoracic fascia in the chest, and lying on each 

 side of the trachea in the neck. 



Although described usually as a single gland, it consists actually of 



* In the description of this gland I have adhered closely to the history of 

 it given by Sir Astley -Cooper, in his beautiful monograph " On the Anatomy of 

 the Thymus Gland," 1832. 



