THE THYMUS 1319 



weigh from .01 to .1 gm. with an average of .035 gm. From their situation they 

 have been divided into a superior, or internal, derived from the fourth branchial 

 pouch, and an inferior, or external, derived from the third branchial pouch. 



The superior parathyreoid glands (fig. 1073) are found, as a rule, on the dorsal surfaces of 

 the lateral lobes of the thyreoid gland at about the junction of the cephalic and middle thirds 

 Occasionally they maj' be situated in the areolar tissue at the level of the apex of the thjTeoid 

 gland or cephalic to it. They may be ventral to the prevertebral layer of the cervical fascia, on 

 the dorsal wall of the oesophagus or pharynx and close to the dorsoniedial margin of the thy- 

 reoid gland. They may also be placed at the level of the caudal border of the cricoid cartilagel 

 rarely as high as the inferior cornu of the thyreoid cartilage or as low as the sixth trachea, 

 ring. Sometimes they are imbedded completely in the thyreoid gland. As a rule, they are 

 tightly attached to the capsule of the thj-reoid gland or situated between its layers. 



The inferior parathyreoid glands (fig. 1073) are less constant in their situation than the 

 superior. They usually are found in relation with the dorsal surface of the lateral lobes of the 

 thyreoid glands, not far from their bases. They may be quite outside the region of the thy- 

 reoid gland along the carotid arteries or the sides of the trachea, or they may be placed more 

 cephalically than usual or extend caudal to the gland as far as the tenth tracheal ring, even into 

 the thorax. They are imbedded, when caudally placed, in fatty areolar tissue in relation with 

 the apex of the thymus gland and the inferior thyreoid veins or applied against the cesophagus. 



The parathyreoids are intimately related to branches of the inferior thyreoid artery, a 

 separate branch of which supplies each of them. When there is a large branch of the inferior 

 thyreoid artery anastomosing with the superior they are more or less in line with this. 



Each parathyreoid gland is surrounded by a fibrous capsule from which extremely vascular 

 septa and trabeculae penetrate into the gland separating and binding together the masses of 

 polyhedral cells which are arranged in solid groups or intercommunicating cords of varying 

 sizes and shapes. 



The cell cords, as a rule, are not arranged like the thyreoid vesicles. At times the secretion 

 may accumulate and produce a vesicular appearance and the secretion then closely resembles 

 colloid. Two kinds of cells, oxyphile and principal cells, have been described; but the inter- 

 mediate forms suggest that these are the same sort of cells in different stages of functional 

 activity. The blood-vessels are distributed in the connective tissue of the trabecule and thus 

 their sinusoids are brought into close connection with the cells of the gland. The nerves are 

 also distributed along the septa. In the highly vascular connective tissue between the cell 

 cords fat cells are foimd separate or in groups. 



The number of parathyreoid glands found by different investigators varies. The average 

 number in a series of cases is less than four. Whether this is due to a real absence of the 

 glands or to failure to find them due to their aberrant location, their inclusion in the thyreoid 

 gland, or the fusion of two glands, is not clear. In some cases it is the superior glands, in 

 other cases the inferior glands, which appear to be missing. On the other hand various com- 

 petent observers have reported finding more than four parathyreoid glands. Five or six are 

 occasionally found; as many as eight have been recorded in one instance. In these cases the 

 number on a side may not be symmetrical. The increased number may be due to the separation 

 of buds in the course of development. The parathyreoid glands are liable to be associated with 

 accessory thymus masses, with small lymphatic glands, and with fat lobules; and as they may 

 somewhat resemble each of these, they may be mistaken unless a microscopic examination is 

 made. 



Blood-supply. — Each parathjnreoid gland is suppUed by a single separate artery derived, as 

 a rule, from one of the glandular, muscular, or oesophageal branches of the inferior thyreoid 

 artery or from the anastomosing branch between the superior and inferior thyreoid arteries. 

 When the glands are in aberrant positions their arteries may be derived from the nearest source. 

 The arteries are distributed along the trabeculse and sep'ta. The veins returning the blood 

 either follow the arteries or they pass to the surface of the gland where they break up into a 

 plexus of thin-walled vessels. Upon leaving the gland the veins empty into some one of the 

 branches of the thyreoid veins. 



Development. — The parathyreoids (epitheUal bodies) begin as prohferations of the epi- 

 thelium on the oral and lateral walls of the dorsal diverticulum of the third and fourth pharyn- 

 geal pouches. The cells show early a histological differentiation with vacuolated and 

 reticulated plasma. The common pharyngo-branchial ducts dimmish in size and become 

 constricted off and separated from the phar3Tix. The parathyreoid glands later become 

 independent and separated from the thymus anlages. The epithehal cells grow out in the 

 form of cords separated by connective tissue and in intimate relation to the blood-vessels. 

 Different kinds of cells are not distinguishable until postfoetal life when evidence of secretion 

 begins. 



THYMUS 



The thymus is a transitory organ of epitheUal origin, but in structure resem- 

 bling the lymphoid tissue. Its function is not clearly understood but it seems to be 

 intimately associated with the growth and nutrition of the individual, and it is 

 classed with the ductless glands of internal secretion. 



It is situated in the ventro-cephalic part of the thorax and extends into the 

 caudal part of the neck (fig. 1074). It lies between the two pleural sacs ventral to 

 the heart and great vessels, dorsal to the sternum and the sterno-thyreoid and 

 sterno-cleido-mastoid muscles. 



