556 THYMUS GLAND. 



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 tym- 

 pani 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 imperfect in the infant, as may be inferred 

 from the small capacity of the nasal fossse, 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 special 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 up- 

 wards from the fourth rib as high as the thyroid gland, resting against 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 

 two lateral, almost symmetrical glands, connected with each other by are- 

 olar tissue only, and having no structural communication : they may there- 

 fore be "properly called, a right and left thymus gland." 



Between the second and third months of embryonic existence, the thy- 

 mus is so small as to be only "just perceptible ;" and continues gradually 

 increasing with the growth of the foetus until the seventh. At the eighth 

 month it is large ; but, during the ninth, it undergoes a sudden change, 

 assumes a greatly increased, size, and at birth weighs 240 grains. After 

 birth it continues to enlarge until the expiration of the first year, when it 

 ceases to grow, and gradually diminishes, until at puberty it has almost 

 disappeared. 



The thymus is a conglomerate gland, being composed of lobules dis- 

 posed in a spiral form around a central cavity. The lobules are held 

 together by a firm areolar tissue (" reticulated"), and the entire gland is 

 enclosed in a coarse areolo-fibrous capsule. 



The Lobules are very numerous, and vary in size from that of the head 

 of a pin to a mo derate- sized pea. Each lobule contains in its interior a 

 small cavity, or " secretory cell" and several of these cells open into a 

 small "pouch" which is situated at their base, and leads to the central 

 cavity, the c reservoir of the thymus" 



The Reservoir is lined in its interior by a vascular mucous membrane, 

 which is raised into ridges by a layer of ligamentous bands situated be- 



* 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. 



